Peptide synthesis: solid-phase method, solution-phase method, and production methods

Peptide synthesis is a controlled process of creating amino acid chains with a specific sequence. In chemical synthesis, selected amino acids are linked using peptide bonds, while simultaneously controlling other reactive groups present in these molecules. The goal is therefore not just to join amino acids, but to obtain a specific molecular structure with a defined sequence, stereochemistry, chain-end architecture, and—if required—additional structural features.

The main methods include solid-phase peptide synthesis (SPPS), classical solution-phase synthesis, and newer liquid-phase peptide synthesis (LPPS) methods using soluble supports or molecular tags. Research laboratories and manufacturing facilities can also combine different approaches, prepare shorter fragments separately and then join them together, or use biological expression if it is more suitable for a given sequence.

Comparison of chemical synthesis with bacterial expression of polypeptides consisting of 20–40 amino acid residues demonstrates an important principle: the practical choice of the production method depends on the specific molecule, its sequence, propensity for aggregation, required modifications, production scale, and analytical requirements. [1]

Obtaining the intended amino acid sequence is only one of the stages of peptide production.

The entire process may include:

sequence design → preparation of building blocks → protection of functional groups → coupling → deprotection → chain assembly → cleavage or fragment condensation → final structural shaping → purification → identity confirmation → impurity analysis → potency determination → final characterization.

For some peptides, additional steps such as cyclization, disulfide bond formation, folding, conjugation, or other chemical modifications are also necessary.

For this reason, the correct assembly of the chain does not automatically mean that the final material has the proper identity, purity, stereochemistry, conformation, stability, or other required characteristics.

What is peptide synthesis?

Creating a specific amino acid sequence

Peptide consists of amino acid residues joined by peptide bonds.

The goal of peptide synthesis is to create these bonds in a predetermined order, often while simultaneously controlling:

• types of amino acids;
• amino acid sequence order;
• stereochemistry;
• N-terminal structures;
• C-terminal structures;
• side chain modifications;
• disulfide bond distribution;
• cyclization;
• of other required structural features.

The sequence is of fundamental importance because an identical amino acid composition can lead to the formation of different molecules if the amino acids occur in a different order.

Stating that a given molecule is „a peptide composed of five amino acids” therefore provides very little information about its actual structure. To properly describe the molecule, one must know the type of amino acids, their sequence, stereochemistry, end groups, and any potential additional modifications.

Two peptides can therefore have the exact same amino acid composition and yet be different molecular structures.

Additional differences may result from chain end modifications, the presence of disulfide bridges, a cyclic backbone, side chain alterations, or different stereochemical configurations.

Chemical peptide synthesis typically uses amino acids whose reactive groups have been selectively protected.

Temporary protecting groups enable the formation of a specific intended chemical bond while limiting competing reactions at other sites of the molecule.

In the subsequent planned stages of synthesis, these groups are removed or modified.

Controlled reaction instead of spontaneous merging

In basic chemical descriptions, the formation of a peptide bond is often presented as a condensation reaction in which two amino acids combine with the net release of a water molecule.

Such an equation describes the overall reaction balance.

However, it should not be equated with the way in which controlled peptide synthesis is carried out in practice.

Simply mixing free amino acids would not allow to reliably determine:

• which amino group is to react;
• which carboxylic group is supposed to react;
• which amino acid should be first;
• which is to be the second;
• will side chains start reacting;
• will the stereochemistry be retained;
• will numerous undesired products not be formed.

Therefore, controlled strategies of activation and protection of functional groups are used in practical chemical synthesis.

A group derived from a carboxyl group is typically activated in such a way that it can react efficiently with a chosen amino group. Protecting groups, reaction sequence, carrier chemistry, coupling reagents, solvent environment, and isolation methods work together to direct the reaction toward the intended product.

Effective peptide synthesis therefore requires control over both the chemistry that is to take place and that which must be prevented.

A good example is the formation of aspartimide.

Researchers developed an alternative method for protecting the carboxyl group, aimed at limiting this problematic side reaction during peptide assembly. [2]

The study shows a broader principle of peptide chemistry: preventing undesired molecular transformations can be just as important as forming the correct peptide bond.

When can a peptide be considered ready?

Finishing the creation of the linear peptide backbone does not always mean the end of the entire synthesis.

Depending on the target molecule, additional steps may be required after chain assembly.

They may include:

• complete removal of protective groups;
• cleavage of the peptide from the solid support;
• removal of temporary markers;
• cyclization;
• oxidation;
• formation of disulfide bonds;
• folding;
• conjugation;
• chain end modification;
• purification;
• analytical characterization.

Some peptides require relatively few additional structural modifications after chain assembly.

For others, the later stages are crucial.

A good example is the synthesis of bovine pancreatic trypsin inhibitor (BPTI). The researchers assembled the linear sequence, purified the material, and then obtained a folded structure containing disulfide bonds. According to the analyses and functional comparisons presented in the study, the final material with the natural sequence corresponded to the natural protein within the range of the studied parameters. [3]

This example shows the difference between the assembly of the peptide backbone and the production of a fully characterized final molecular form.

A method that allows obtaining the correct linear sequence does not automatically confirm:

• proper folding;
• proper formation of disulfide bonds;
• biological activity;
• stability;
• pollution profile;
• fitness for a particular purpose.

Each of these issues requires separate evidence.

Chemical synthesis, biological expression, and combined methods

In chemical peptide synthesis, previously prepared molecular blocks are joined in controlled laboratory reactions.

Biological expression, on the other hand, utilizes cellular machinery to produce the encoded amino acid sequence. Subsequently, the material must be recovered, purified, and, if necessary, subjected to further processing.

Both methods can lead to the production of peptides or polypeptides, but they involve different possibilities and limitations.

Chemical synthesis can be particularly useful when needed:

• selected non-standard amino acids;
• site-specific chemical modifications;
• specific chain end structures;
• isotope tracers;
• peptide backbone modifications;
• other traits difficult to obtain through standard biological expression.

Biological expression can be attractive in the case of suitable longer sequences or specific labeling strategies, but it does not eliminate problems occurring at later stages of production.

Material recovery, folding, processing, host-related impurities, purification, and analytical characterization must still be taken into account.

Therefore, none of the methods is universally the best.

In the comparison of polypeptides consisting of 20–40 amino acid residues, both sequence length and its propensity for aggregation affected the results of solid-phase synthesis and bacterial expression. [1]

These observations support an approach where the production method is selected for a specific molecule.

Historical data on performance, costs, equipment, and production time should not be treated as universal modern values, because technologies, reagents, automation, purification methods, and production economics change with technological development.

Solid-phase peptide synthesis

The growing chain is attached to an insoluble support

In solid-phase peptide synthesis (SPPS), the growing peptide chain remains attached to an insoluble support during successive synthesis cycles.

The carrier is most often a functionalized polymer resin.

A linker binds the peptide to the carrier and influences, among other things, the manner in which the final chain is subsequently released and the type of end group obtained.

One of the main practical advantages of SPPS is easier material separation.

Upon completion of a specific reaction step, soluble reagents and many soluble by-products can be removed by washing, while the growing peptide remains bound to the solid phase.

Because of this, there is no need to isolate and purify the growing peptide as a separate, free compound after the addition of each successive amino acid.

However, this convenience can lead to a significant misunderstanding.

Washing the resin does not mean that all incorrect peptides have been removed.

If the chain attached to the resin does not react during one coupling step, it may still remain on the support.

In subsequent cycles, it may react again and elongate further.

As a result, a deletion sequence may be formed, which is a molecule differing from the target peptide by the absence of one or more amino acid residues.

Similarly chemically modified or incompletely unblocked chains bound to the resin can participate in subsequent synthesis steps.

Therefore, SPPS simplifies the separation of soluble substances from the resin-bound material.

However, it does not guarantee perfect sequence alignment.

A repeating cycle of uncoupling and coupling

A typical stepwise SPPS synthesis involves repeating three basic operations:

disengaging → coupling → flushing.

First, the reactive amino group located at the end of the growing chain is uncovered.

Next, another protected amino acid is attached to the chain.

Soluble reagents, excess amino acid, and many reaction products are removed before the next cycle begins.

Repeating this process makes it possible to build the peptide one amino acid residue at a time.

In classical SPPS, the C-terminal residue is usually attached to the support first, and then the chain is elongated towards the N-terminus.

This differs from standard ribosomal protein synthesis, during which successive amino acids are added to the C-terminus of the growing chain.

However, the final amino acid sequence can be identical even though the direction and chemistry of the folding process are different.

The term „cycle” should not be understood as a single universal set of reaction conditions suitable for every residue and every sequence.

The course of the reaction may depend on:

• protective group strategies;
• amino acid type;
• spatial conditions around the reaction site;
• resin type;
• Linker;
• chain lengths;
• aggregation;
• solvent;
• coupling chemistry;
• structural behavior of the growing peptide.

A reaction that proceeds easily at the beginning of the synthesis can become significantly more difficult at a later stage.

Therefore, process monitoring remains important even in highly automated systems.

Why do resin properties matter?

A durable medium is not merely a passive holder.

Its physical and chemical properties can directly affect the course of the reaction.

One of the important parameters is swelling.

If the resin swells properly in the chosen solvent environment, reagents can more easily reach the reactive sites located throughout the polymer structure.

Insufficient swelling or structural changes in the resin may limit this access.

The following may also be important:

• stocking density;
• polymer architecture;
• linker script;
• solvent compatibility;
• mechanical stability;
• porosity;
• diffusion properties;
• interactions between growing peptide chains.

As the peptide length increases, individual chains can also interact with each other.

Aggregation on the resin can restrict access to reactive sites and lead to incomplete coupling or incomplete deprotection.

This means that a sequence that behaves correctly at the beginning of the synthesis may become increasingly difficult as the chain lengthens.

Researchers have developed a variable-bed flow reactor that enabled monitoring of resin swelling based on pressure-related measurements. [4]

The study focused on resin aggregation and incomplete bonding as process issues that might otherwise be difficult to directly observe during synthesis.

This shows that monitoring the physical behavior of the medium can provide information that goes beyond merely recording the added reagents.

What is Wang resin used for?

Wang linker chemistry is widely used in SPPS when a C-terminal carboxylic acid is desired after peptide cleavage.

The peptide is attached via an ester-type bond, which can later be cleaved under appropriate conditions.

Other linker systems may be selected when a different C-terminal structure is required, for example an amide.

It is worth distinguishing the linker chemistry from the entire physical carrier.

Wang-type linkers can be incorporated into various polymer architectures.

Such materials do not have to have identical:

• swelling properties;
• capacities;
• solvent compatibility;
• mechanical properties;
• behaviors during a reaction.

Studies using peptidyl-Wang-TentaGel resins demonstrated a specialized alternative method for obtaining peptide hydrazides, which are useful intermediates in peptide chemistry. [5]

This does not mean that standard cleavage from Wang resin usually leads to the formation of peptide hydrazides.

It shows, however, an important principle:

support + linker + reaction conditions + cleavage chemistry together determine the resulting product.

SPPS synthesis in batch and flow systems

SPPS can be carried out in conventional batch vessels or in systems where reagents flow through a reactor containing a solid support.

Flow systems can enable rapid reagent exchange and more precise control of the chemical environment surrounding the resin.

A 2014 study presented rapid flow-based peptide synthesis with continuous monitoring and applied it to several peptides and a small protein. [6]

A later automated study described significantly faster chain assembly, yielding crude product purity and isolated yields comparable to classical batch synthesis for the analyzed examples. [7]

These studies show the capabilities of specific systems under defined conditions.

They do not prove, however, that every peptide can be produced equally quickly and with the same quality.

The time required to produce the peptide also involves much more than just chain assembly.

The entire process may include:

• reagent preparation;
• resin preparation;
• coupling;
• disengaging the safety;
• rinsing;
• schism;
• product processing;
• purification;
• solvent removal;
• final structural shaping;
• analytical tests;
• documentation.

Rapid binding is therefore only one element of overall production efficiency.

Liquid-phase synthesis

What does this term mean?

The term „solution-phase peptide synthesis” can be used in several senses.

In a broad sense, it may simply mean peptide synthesis carried out in solution.

However, in more specialized literature, a distinction is often made between:

1. classical solution-phase peptide synthesis;
2. solid-phase peptide synthesis;
3. newer LPPS methods, in which the growing chain possesses a soluble tag or support.

This distinction is important because the method of product separation and recovery changes.

A freely dissolved, protected peptide intermediate may require isolation utilizing its own physical and chemical properties.

In the case of a labeled intermediate, the label can be designed to facilitate precipitation, extraction, membrane retention, or another method of separation.

The review on LPPS clearly distinguishes between classical solution-phase synthesis, SPPS, and methods utilizing soluble tags. [8]

The publication is useful for organizing terminology and methodology.

However, it should not be interpreted as a single experiment proving that one method is the best for every peptide.

Classical solution-phase peptide synthesis

In classical solution-phase synthesis, protected amino acids, peptide intermediates, or fragments remain dissolved in the reaction medium instead of being covalently attached to an insoluble resin.

Upon completion of the reaction, the corresponding intermediate can be isolated, purified, and, if necessary, characterized before initiating the next step.

This may bring certain benefits.

Researchers can:

• characterize individual intermediates;
• tailor the purification method to a specific intermediate;
• optimize individual transformations;
• prepare fragments independently;
• modify the synthesis route when a specific intermediate causes problems.

However, solution-phase synthesis can also be associated with significant difficulties.

As the peptide chain lengthens, they can:

• become poorly soluble;
• aggregate;
• be difficult to crystallize;
• to generate closely related impurities;
• hard to separate;
• require multiple isolation;
• cause material losses during subsequent processing stages.

Solution-phase synthesis is therefore not automatically simpler or easier to scale than SPPS.

Its utility depends on the specific sequence, intermediate stability, solubility, length of the synthesis route, purification behavior, and the efficiency of subsequent isolation steps.

The development of labeled LPPS strategies stems in part from attempts to solve these very problems related to separation and material handling. [8]

Soluble tags bridge reaction design and separation design

In labeled LPPS, the growing peptide possesses a temporary molecular attachment designed in such a way as to influence its physical properties.

The tag may enable maintaining sufficient solubility of the peptide during the reaction and then facilitate its selective recovery.

Depending on the technology, separation can use:

• precipitation;
• extraction;
• retention on the membrane;
• phase behavior;
• other properties resulting from the presence of the tag.

One of the experimental strategies used a long-chain C-terminal protecting group to prepare peptide amides.

Repeated reactions and precipitation steps enabled the researchers to prepare a hydrophobic peptide, which, in the comparison performed by them, was difficult to obtain satisfactorily using the SPPS method. [9]

This demonstrates the usefulness of this specific strategy for the analyzed sequence.

However, it does not prove the universal superiority of tagged synthesis.

The tag becomes part of the manufacturing process and raises its own questions:

• How effectively is it connected?
• Does it change the course of the reaction?
• How can a labeled semi-finished product be reliably recovered?
• Does it lead to the creation of new pollution?
• How is it removed?
• Can complete deletion be confirmed?
• Which analytical methods confirm its absence in the final product?

Dissolvable markers should therefore be treated as functional elements of the process design, rather than merely as simple labels.

Comparison of main methods

The comparison presents the general principles of operation of the individual methods.

It does not specify which of them will provide the best performance, cleanliness, cost, environmental sustainability, or scalability for a particular sequence.

These parameters should be evaluated based on the actual molecule and the specific manufacturing process. [4] [8] [9]

Fragment joining and hybrid production

A peptide does not always have to be created in a single uninterrupted process consisting of the addition of successive amino acid residues.

An alternative strategy is to separately prepare shorter peptide fragments and then join them.

For example, a hybrid process can use SPPS to obtain individual fragments and solution-phase chemistry to ligate them.

Fragment-based strategies can limit some of the problems associated with very long multi-step synthesis.

However, they introduce new challenges, such as:

• selection of the appropriate connection location;
• solubility of fragments;
• activation of the junction site;
• chemoselectivity;
• retention of stereochemistry;
• side effects;
• fragment purification;
• purification after their combination.

An important example of controlled fragment joining is native chemical ligation.

In the classical version, a peptide thioester reacts with a second peptide containing an N-terminal cysteine. A subsequent rearrangement leads to the formation of a native peptide backbone bond at the ligation site.

Initial research demonstrated the possibility of obtaining, after ligation and folding, a protein containing disulfide bonds. [10]

This confirms the importance of this fragment-linking strategy.

However, this does not mean that any two unmodified peptide fragments can spontaneously join together.

Coupling and protecting groups

Coupling creates an intended connection

Coupling is a reaction stage during which a new peptide bond is formed between selected units.

Usually, the group derived from the carboxyl group is activated so that it can effectively react with the corresponding amino group.

The activation chemistry must be compatible with:

• increasing sequence;
• temporary protective groups;
• side-chain protection;
• stereochemistry;
• carrier or marker;
• solvent environment;
• with other functional groups present in the molecule.

Effective coupling is particularly important in stepwise synthesis because incomplete reaction can lead to the formation of deletion sequences.

If a single amino acid addition step fails for some of the growing chains, such chains may remain in the system and participate in subsequent cycles.

As a result, molecules very similar to the target peptide, but lacking a single residue, may be formed.

Even minor inefficiencies can therefore accumulate over many synthesis steps.

A simple mathematical example illustrates this problem well.

If each of the 50 independent assembly stages had a 99% probability of success, the percentage of molecules for which all stages would be successful would be:

0.99^50 ≈ 0.605

that is about 61%.

This is exclusively a mathematical illustration.

It does not constitute a prediction of the actual purity or synthesis yield of a 50-residue peptide.

Actual production includes:

• different coupling efficiencies in individual positions;
• steps to disarm;
• side effects;
• sequence-dependent difficulty;
• re-engagement strategy;
• cleavage losses;
• purification losses;
• other process variables.

The calculation shows only why even a seemingly minor inefficiency of a single step can take on great significance in a multi-step synthesis.

Protecting groups control the reaction site

An amino acid can possess several chemically reactive groups.

Without selective control, coupling reactions could occur at undesired sites.

Protecting groups temporarily block selected reactive sites.

The amino group used for chain elongation typically requires temporary protection, whereas reactive side chains may require protection that remains in place throughout multiple synthesis cycles.

The connection to the carrier must also survive these stages until the planned separation.

The choice of protecting groups therefore affects the entire synthesis strategy.

The following must be taken into account:

• the method of introducing them;
• stability;
• removal method;
• compatibility with other protective groups;
• compatibility with the medium;
• side effects;
• impact of a specific sequence;
• final disarming conditions.

Studies utilizing cyanosulfurylid-based protection focused on the formation of aspartimide by altering the protection method of the carboxyl group. [2]

The results show that the protecting group strategy influences not only whether the intended reaction can take place, but also which side reactions become possible.

Fmoc, Boc and orthogonal protection

Fmoc and Boc belong to widely used amino-protecting groups.

They differ significantly in the way they are removed.

Fmoc is typically removed under basic conditions, whereas Boc is sensitive to acidic conditions.

These properties allow them to be used as elements of broader protection strategies.

In popular Fmoc-based SPPS, the temporary Fmoc N-terminal protecting group is repeatedly removed, while selected side-chain protecting groups and the resin linkage remain intact for later stages.

Other strategies may use different combinations.

A particularly important concept is orthogonal protection.

Two protecting groups are considered orthogonal if one can be selectively removed under conditions in which the other remains sufficiently stable to allow further synthesis to proceed.

Orthogonality is therefore a practical relationship of chemical compatibility.

It does not mean that every protected group will remain completely unchanged after an unlimited number of exposures to all reaction conditions.

Side effects can still occur.

Fmoc synthesis studies have described both aspartimide formation and undesired stereochemical changes despite the assumed selectivity of the general protection strategy. [11]

Stereochemistry is part of the identity of a molecule

Many amino acids have a chiral center.

This means that their atoms can adopt different spatial arrangements corresponding to different stereochemical forms.

Changing the configuration of a single stereogenic center can lead to the formation of another peptide structure without the need to change its nominal mass.

Therefore, the mass alone cannot confirm the full identity of the peptide.

Epimerization usually means a change in the configuration of one stereogenic center in a molecule containing several such centers.

Racemization is also often discussed in peptide chemistry in the context of the loss of stereochemical integrity of the amino acid residues.

These changes are significant because molecular recognition can strongly depend on the spatial arrangement of the atoms.

Two molecules with the same elemental composition and nominal mass can therefore behave differently if they differ in stereochemistry.

In a study of microwave-assisted Fmoc synthesis, it was found that some residues were susceptible to undesirable stereochemical changes under the conditions employed. Modifying the method reduced the occurrence of these reactions.[11]

This does not mean that heating, by definition, damages all peptides.

The correct conclusion is that the acceleration of the reaction should be evaluated together with the sequence-dependent chemical selectivity.

Aspartame and other related pollutants

The formation of aspartimide is a known side reaction related to certain sequences containing aspartic acid.

An undesirable cyclic intermediate may be formed, from which subsequently compounds with altered backbone connectivity or stereochemistry are formed.

The probability of this reaction depends on the sequence and the conditions of the process.

Other impurities associated with synthesis can result from:

• incomplete integration;
• incomplete deactivation;
• the formation of deletion sequences;
• reaction of side chains;
• stereotypical changes;
• reactions during deattachment;
• oxidation;
• reorganisations;
• conditions of product processing;
• incomplete removal of protective groups.

Some pollutants cause a readily noticeable change in the molecular mass.

Others are much more difficult to distinguish.

In analytical studies of a synthetic peptide that is a substrate of renin, many impurities have been identified, including components exhibiting the same mass change but containing modifications at different positions.[12]

An additional structural analysis was needed to distinguish them.

The researchers also considered the proposed mechanisms for the formation of these pollutants as hypotheses.

This is an important distinction:

Determining what pollution is and proving the exact mechanism of its formation are two separate scientific questions.

The probable mechanism should not be presented as confirmed solely because it corresponds to the observed molecular mass.

Purification, scaling, and quality of production

Raw product, purified product and yield

The crude peptide is the material obtained before the completion of the appropriate purification steps.

May contain:

• target peptide;
• deletion sequences;
• material not completely secured;
• modified sequences;
• residues of reagents;
• products related to de-pollution;
• salt;
• solvents;
• other components formed during the process.

Purification aims to separate the appropriate peptide from this mixture.

It is necessary to clearly distinguish three concepts:

purity of the raw product — measured composition of the material before final purification;

the yield of the purified or isolated product — the amount of recovered target material relative to a specific initial or theoretical value;

final purity — the composition of the isolated material determined using a specific analytical method.

These concepts are not interchangeable.

The process can lead to the formation of a large amount of raw material, but it allows the recovery of a relatively small amount of purified product.

Another process may yield less raw material, but it will be characterized by high selectivity in creating the final product.

Similarly, high analytical purity does not necessarily mean high production efficiency.

In accelerated flow synthesis studies, the purity of the raw material and the yield after isolation were correctly reported separately [7].

Chromatography separates components, but it does not answer every question.

High-performance reverse-phase liquid chromatography (RP-PLHPLC) is widely used for the analysis and purification of peptides.

Different molecules interact in different ways with the stationary and moving phases.

Under appropriate conditions, these differences allow the components of the mixture to be separated.

Analytical and preparative chromatography have related but different applications.

Analytical HPLC is primarily used to analyze the composition of the sample.

Precipitative HPLC is used to physically collect the separated components.

The chromatogram can provide many valuable information.

However, a large main peak or a high percentage of its surface area should not be considered as a complete molecular characteristic.

The results of chromatography depend, among other things, on:

• type of column;
• the moving phase;
• gradient;
• temperatures;
• detection methods;
• wavelength;
• sample preparation;
• the way pixels are integrated;
• resolution;
• properties of pollutants.

Two compounds can potentially elute at the same time.

Some sample components may also react differently to the detector used.

Therefore, high surface purity in HPLC is not synonymous with full confirmation of molecular identity.

Isotopic studies of peptide impurities and chiral analyses show why additional analytical methods may be needed. [12] [13]

The analytical method must correspond to a specific structural question.

Identity, purity, and content mean different things

In the description of peptides, three concepts are particularly important.

The identity answers the question:

Is the intended molecule present in the sample?

Purity answers the question:

What other components are present in the sample or which part of the measured signal corresponds to the main component?

The content answers the question:

How much real peptide is there in the sample?

These questions cannot always be answered with a single measurement.

The material containing the peptide may also contain:

• water;
• hypotenuses;
• salt;
• solvent residues;
• other components related to the process;
• pollution.

The total mass of the bold material does not therefore have to correspond to the mass of the peptide itself.

Similarly, the agreement between the measured molecular mass and the theoretical value does not confirm:

• complete amino acid sequence;
• proper connection of the individual parts;
• stereochemical integrity;
• disulfide bond distribution;
• folding;
• absence of isobaric impurities.

Mass spectrometry can provide strong evidence regarding the molecular mass.

Tandem mass spectrometry and other structural techniques can provide additional information about the sequence.

Chiral methods allow us to analyze stereochemical issues that cannot be resolved by conventional mass measurements.

One analytical study even included the racemization that occurs during sample preparation. This illustrates another important principle: the analytical procedure itself can produce artifacts if it is not properly designed. [13]

Twisting and activity may require separate confirmation

In the case of structurally limited peptides and small proteins, the correct sequence and molecular mass may still be insufficient to confirm the intended final structure.

Additional questions may include:

• Have the appropriate disulfide bridges been formed?
• Has the molecule adopted the expected conformation?
• Is the cyclization complete?
• Are there structural isomers present?
• Does the synthetic molecule exhibit the expected activity in a specific experimental test?

Several analytical and biophysical methods were used in the BPTI synthesis study, along with an assessment of trypsin inhibition.[3]

The combined results supported the consistency of the synthetic material with a natural sequence in terms of the criteria examined.

The activity test, however, answers a specific functional question.

It does not replace the analysis of pollutants.

Similarly, similar activity in one test does not confirm full equivalence of all biological, structural, or production properties.

Scaling up production changes more than just the amount of material

Increasing the scale of production does not solely consist in a proportional increase in the quantity of all the reagents.

Physical processes can behave differently on a larger scale.

They may have significance:

• mixing;
• heat exchange;
• distribution of reagents;
• diffusion;
• thickening of the resin;
• pressure;
• filtration;
• the solvent economy;
• precipitation;
• solubility of semi-products;
• mass transport;
• the capabilities of the purification system.

These factors can influence both the course of the reaction and the formation of pollutants.

Cleaning at later stages can also become a bottleneck for the entire production.

Increasing the synthesis rate is of limited significance if the purification system is unable to effectively separate the contaminants that are produced or to recover the product of the required quality.

In an industrial analysis covering 40 peptide synthesis processes from 14 pharmaceutical companies, the consumption of materials during synthesis, purification, and isolation was evaluated.[14]

The study shows why the evaluation of production should be conducted for the entire process.

However, this does not mean that every factory, sequence, or production technology achieves identical results.

The quality of the production is the result of many control elements

Reliable production of peptides requires more than just obtaining a single satisfactory analytical result.

A controlled process usually involves several related elements:

Defined target molecule → appropriate starting materials → controlled synthesis → controlled purification → appropriate sampling → validated or target-specific analysis → batch documentation.

The party’s documentation combines analytical results with material that was actually produced.

The Certificate of Analysis (COA) summarizes the selected test results of a specific batch.

Its informational value depends on:

• the type of tests performed;
• the methods used;
• the usefulness of these methods;
• specification;
• identification of the sample;
• the possibility of tracing the party;
• confirmation that the reported results actually apply to the material in question.

A single value defined as „purity” cannot by itself confirm:

• complete molecular identity;
• the content of the peptide;
• stereochemical integrity;
• level of solvent residues;
• water content;
• the composition of the counterions;
• microbiological properties;
• folding;
• biological activity;
• suitability for a specific purpose.

The appropriate set of analyses depends on the intended use of the material.

Peptides used as an analytical reference material, research reagents used in a cellular experiment, and materials in the pharmaceutical development phase may require answers to other qualitative questions.

Analytical evidence should therefore be interpreted within a clearly defined context, and not as universal proof of quality.

The environmental balance assessment must cover the entire process

The synthesis of peptides may require significant amounts of solvents and reagents.

The environmental impact assessment should therefore take into account much more than just the coupling stage itself.

Important elements may include:

• preparation of building blocks;
• coupling agents;
• deactivator reagents;
• rinsing;
• schism;
• product processing;
• chromatography;
• replacement of solvents;
• isolation;
• drying;
• waste management.

Reducing the environmental impact may require a simultaneous improvement of several stages.

The published analysis concerning peptide production discusses the problems associated with both synthesis and purification, and indicates the need to improve the entire process design.[15]

As a prospective publication, it does not constitute a controlled, direct comparison proving that one specific technology is always more environmentally friendly.

Similarly:

faster synthesis does not automatically mean more environmentally friendly synthesis,

and the lower consumption of solvents during the synthesis itself does not automatically mean a smaller impact of the entire production process.

Environmental comparisons require the use of consistent analytical limits and should ideally relate the consumption of resources to a specific amount of the isolated and characterized final product.

Frequently asked questions

1. How is the synthesis of peptides at the solid phase different from the synthesis in the liquid phase?

In continuous phase synthesis, the growing peptide remains attached to the insoluble carrier, whereas in classical synthesis, the peptide intermediate remains dissolved and usually has to be recovered based on its physical and chemical properties. In a narrower sense, an LPPS dissolved growing chain additionally has a soluble label or carrier that facilitates subsequent separation. Studies of labeled synthesis show one of the strategies of combining the solution chemistry with recovery by extraction, whereas the literature on LPPS distinguishes such solutions from both classical synthesis in solution and SPPS. No method is by definition optimal for every peptide. The fundamental difference concerns the physical handling and separation of the growing chain during the subsequent steps of synthesis. [8] [9]

2. Why can impurities form during peptide synthesis?

Contaminations can arise because intended reactions do not always occur quantitatively, and other functional groups may participate in undesirable reactions. Incomplete coupling can lead to deletion sequences, incomplete deprotection can disrupt further chain assembly, and some sequences are susceptible to stereochemical changes, the formation of aspartimide, oxidation, rearrangement, or byproducts associated with deprotection. Studies of Fmoc synthesis and structural analysis of synthetic peptide contaminants document several such examples.[11][12] The frequency and significance of individual contaminants depend on the sequence and process, therefore automation of synthesis should not be equated with complete absence of chemical errors.

3. What role does the resin play in the synthesis of peptides?

The resin maintains the growing peptide while soluble reagents and many byproducts can be removed between successive synthesis steps. This greatly simplifies the repeated chain assembly, but the resin is not solely a mechanical hold. Its swelling, the degree of occupancy, the polymer architecture, the compatibility with solvents, and the availability of reaction sites can all influence the effectiveness of the reagents reaching the attached peptide. In studies using a variable-complex reactor, the behavior of the resin during aggregation analysis and the efficiency of binding were monitored. [4] Such monitoring does not eliminate sequence errors, but it shows why the physical state of the resin can be an important process variable.

4. What is the purpose of Wanga resin in peptide synthesis?

Wang-type carriers are commonly associated with the preparation of peptides released after appropriate cleavage from the C-terminus in the form of carboxylic acid. However, the exact end product depends on the linker, the carrier architecture, and the applied cleavage chemistry, and not only on the name of the resin. The researchers also presented a specialized method for converting the peptide-Wang-TentaGel material into peptide hydrazides, which can serve as useful synthetic intermediates. [5] The alternative reaction does not change the significance of the standard Wanga-resin cleavage. However, it illustrates why when predicting the product, the entire carrier system and reaction conditions must be analyzed.

5. Does SPPS build a peptide from the N-terminus or from the C-terminus?

Classical stepwise SPPS usually begins with the attachment of the C-terminal residue to the carrier, after which the chain is extended towards the N-terminus. This is the opposite direction to the standard chain extension on the ribosome, where the protein grows by adding consecutive amino acids to the C-terminus. The synthesis of BPTI is an example of using protected building blocks and the carrier to prevent subsequent release, purification, and folding of the product.[3] However, there are specialized peptide chemistry strategies that utilize other solutions, so building from the C-terminus to the N-terminus should be considered a classic SPPS approach rather than an absolute rule for all synthesis methods.

6. Why are security groups needed?

Amino acids often contain several reactive functional groups. Protective groups temporarily prevent the reaction at selected sites, leaving the appropriate site available at a specific stage of synthesis. This helps to control the order and manner in which residues are joined together. Without proper protection, competing reactions could lead to the formation of complex mixtures or products with a modified structure. Studies on the formation of aspartimide showed that a change in the chemistry of the carboxyl group protecting group can limit an important side reaction in the sequences analyzed. [2] Protective groups do not guarantee an ideal course of synthesis, but they provide selective chemical control that must remain consistent with all the important stages of the process.

7. What are the differences between Fmoc and Boc?

Fmoc and Boc are widely used temporary groups protecting the amino group, but they react to other chemical conditions. Fmoc is usually removed in an alkaline environment, whereas Boc is sensitive to acids. This difference affects the way they are linked to side chain protection, linkers, and other stages of synthesis. Synthesis based on Fmoc is widely used in modern SPPS, but studies have also shown the possibility of sequential reaction-dependent side reactions in such conditions.[11] In practice, therefore, it is more useful to compare entire protection strategies and their compatibility with a given sequence than to consider only the names of the two protective groups.

8. What does orthogonal protection mean?

Orthogonal protection means that one protective group can be removed under conditions where another necessary protective group remains sufficiently stable to allow the synthesis to continue. This allows different reactive sites to be revealed at specific stages. This concept is fundamental during controlled synthesis of complex peptides. However, orthogonality is dependent on conditions, not on absolute values: repeated exposure, sequence characteristics, and reaction parameters can still lead to side reactions. Studies of alternative methods of protecting carboxyl groups show how the protection strategy can be modified to control difficult peptide chemistry.[2]

9. What is a delection sequence?

A deleuxin sequence is a peptide lacking one or more residues present in the intended target sequence. It can be formed when the amino acid binding step does not occur completely, but the unreacted chain remains attached to the carrier and participates in subsequent cycles. Because the incorrect chain remains bound to the substrate, conventional elution does not necessarily remove it. Real-time SPPS monitoring studies indicate incomplete amide bond formation as an important process problem.[4] Therefore, a deleuxin sequence is a structural and identity problem of the product, and not merely a residue of the reagent.

10. Can two peptide impurities have the same molecular mass?

Yes. Different molecular structures can have identical or indistinguishable nominal mass. For this reason, the mere mass agreement does not confirm the full identity of the molecule. In studies of a synthetic peptide that is a substrate for renin, isobarytic impurities were identified, with modifications occurring at different positions. Derivatization and tandem mass spectrometry provided additional structural information. [12] This example shows why the observed mass agreement with the expected one cannot independently confirm the correct sequence, connectivity, stereochemistry, or absence of structurally similar isomers.

11. What are epimerization and racemization in peptide synthesis?

Epimerization and racemization involve undesirable changes in stereochemical configuration. They can occur without changing the nominal amino acid sequence or molecular mass; therefore, conventional mass measurements may not show a structural difference. In studies of microwave-assisted Fmoc synthesis, residues susceptible to stereochemical changes were identified and shown that modifying reaction conditions can limit such processes.[11] The risk depends on the building block, sequence, and process conditions. Stereochemical quality therefore requires analytical methods capable of evaluating the configuration, and it cannot be inferred solely from molecular mass.

12. What is the formation of aspartimide?

The formation of aspartimide is an undesirable reaction associated with specific sequences containing aspartic acid. The formation of the cyclic intermediate can lead to subsequent products with altered backbone or stereochemical connectivity, reducing yield and making purification difficult. Researchers analyzed alternative protection strategies designed to reduce this reaction in selected peptides and proteins.[2] The results do not indicate a single universal solution for all sequences containing aspartic acid. They do, however, show why the design of synthesis and contamination control strategies must take into account the specific chemical behavior of a given sequence.

13. Does high purity of HPLC confirm the correct identity of the peptide?

No. The purity of the HPLC describes the sample within the context of a specific chromatographic separation, the detector used, and the way the data are analyzed. A high percentage of the main peak area can be valuable evidence that the sample is dominated by a single component separated by chromatography, but it does not by itself confirm the complete molecular identity. Co-occurring impurities, stereoisomers, and other variants may require additional techniques. In studies of the chiral analysis of peptides, a specially designed process was used that also accounted for the stereochemical changes occurring during sample preparation. [13] Identity and purity should therefore be treated as related but separate analytical questions.

14. Does the performance of the purified product mean the same thing as purity?

No. The yield of a purified or isolated product determines the amount of target material recovered relative to a defined initial or theoretical amount. Purity, on the other hand, describes the composition of the recovered material according to a specific analytical method. Synthesis can lead to the production of a relatively small amount of a product of very high purity or a large amount of raw material containing significant amounts of impurities. In studies of automated flow synthesis, the purity of the raw product and the yield after isolation are reported separately.[7] Separating these parameters prevents the erroneous treatment of a high value of one of them as evidence of a high value of the other.

15. Can long peptides be produced by joining shorter fragments?

Yes. In a fragmental strategy, shorter peptide segments can be prepared separately and then combined using appropriate chemistry. Nuclease chemical ligation was an important example where unprotected peptide fragments were combined in a way that led to the formation of a natural peptide backbone bond, and then a folded protein was obtained. [10] This does not mean that any peptide fragments can spontaneously bind together. The structure of the fragments, the ligation chemistry, the choice of ligation site, solubility, stereochemistry, and purification requirements must be taken into account. Fragment assembly is therefore an alternative synthesis design strategy, not an automatic solution to the problems associated with long peptides.

16. Does faster automated synthesis eliminate the need for purification?

No. Faster automated or flow-based synthesis reduces the time of specific synthesis steps, but does not prove that every resulting molecule has the intended structure. In studies of rapid flow synthesis, the raw material is still characterized and subsequent purification is applied, and in subsequent works on automation, the purity of the raw material and the yield after isolation are compared separately.[6][7] Good system performance for the investigated sequences does not also guarantee identical results for each new target. Accelerated synthesis still needs to be combined with appropriate purification, identity confirmation, impurities analysis, and final characterization.

17. Why is increasing the scale of production more complicated than preparing a larger batch?

Increasing scale changes physical and chemical conditions, not just the amount of material. Mixing, heat exchange, polymer swelling, the distribution of reagents, pressure, filtration, solvent handling, the solubility of semi-finished products and the capabilities of the purification system may behave differently at higher production volumes. In industrial analysis, 40 peptide processes were evaluated separately for synthesis, purification, and isolation during material consumption analysis. [14] The results show why production must be considered a complete process. A scalable method must consistently lead to the production of a characterized material at all stages, and not just a larger amount of crude peptide.

18. Is the peptide ready immediately after the correct sequence is assembled?

Not necessarily. The assembly of a chain creates the backbone of a sequence only to the extent that the process and analytical data confirm it. Depending on the molecule, further purification, deprotection, cyclization, oxidation, formation of disulfide bridges, folding, conjugation, purification, and characterization may still be necessary. The synthesis of BPTI provides a good example of a process in which, after assembling a linear sequence, purification and the formation of a folded structure containing disulfide bridges followed. [3] The term „ready peptide” should therefore refer to the molecular form required in a given scientific context, and not merely to the completion of the sequential addition of amino acids.

19. Does the correct molecular mass confirm the correct peptide sequence?

No. Mass consistency is a valuable evidence, but it cannot independently confirm the complete sequence or molecular structure. Isobaric structures can have the same mass, and differences in stereochemistry may not at all lead to a change in molecular mass. Studies of synthetic peptide impurities have shown that distinguishing structurally different components may require tandem mass spectrometry and additional analyses.[12] In cases involving the structural integrity of stereochemically distinct components, chiral methods may also be needed.[13] Mass consistency must therefore be considered as one of the elements in the assessment of identity, rather than its complete confirmation.

20. Why should purification be considered as part of the peptide production process, and not just an end-product addition?

Purification determines whether the target peptide can be effectively separated from the sequencing products, by-products, residue of reagents, structurally similar impurities, and other components formed during synthesis. A method that allows the very rapid production of large quantities of crude material may still be impractical if the target product of required quality cannot be effectively recovered. For this reason, industrial analyses of peptide processes include synthesis, purification, and isolation together. [14] The efficiency of production should ultimately be evaluated based on the quantity and quality of the isolated final material, rather than solely on the speed of synthesis.

Limitations when comparing peptide synthesis methods

When interpreting studies comparing peptide production technologies, several important limitations should be kept in mind.

First, the sequence strongly influences the difficulty of synthesis. A method that works for one peptide may not work the same way for another. Aggregation, hydrophobicity, amino acid composition, length, tendency to form secondary structures, and susceptibility to side reactions can all change the behavior of the process.[1][4]

Secondly, the purity of the raw material, the yield after isolation, the final purity and the content of peptide are different parameters. Comparing methods solely based on one of them may give an incomplete picture of the production efficiency. [7]

Thirdly, rapid chain assembly does not mean rapid production of the entire material. Debinding, cleaning, isolation, final structuring, analytical testing, and documentation also affect the overall production time. [6] [7]

Fourthly, analytical purity depends on the method used. HPLC, mass spectrometry, tandem mass spectrometry, and chiral analysis address different questions. One analytical result does not necessarily confirm the full structural identity. [12] [13]

Fifthly, scale affects the behavior of the process. Laboratory synthesis results cannot be automatically transferred to industrial production without considering mixing, mass transport, purification capabilities, material consumption, and other scale-dependent factors. [14]

Sixthly, historical economic comparisons may lose their relevance over time. The costs of reagents, the level of automation, expression technologies, purification equipment, solvent recovery, and production infrastructure are constantly evolving. Older cost analyses may still have scientific significance, but they do not necessarily reflect the current economics of the market.[1]

Seventhly, environmental comparisons require consistent limits for analysis. Assessing solvent consumption during the actual chain synthesis while ignoring purification and isolation can lead to misleading conclusions regarding the equilibrium of the process.[14][15]

Finally, the name of the method itself does not determine the quality of the product. SPPS, solution synthesis, LPPS, fragment ligation, and biological expression can all lead to the production of high-quality material if the process is properly designed and controlled. However, none of these methods guarantees automatically the correct identity, purity, stereochemistry, folding, or suitability for a specific application.

Summary

The synthesis of peptides is best understood as a controlled process of molecule production, rather than simply the formation of peptide bonds.

The goal is to build a specific amino acid sequence while controlling the way the residues are joined, the chemistry of the protecting groups, stereochemistry, side reactions, and any additional structural requirements.

The synthesis of peptides at the solid phase maintains the growing chain on an insoluble carrier, making the repeated coupling and washing steps well suited for automation. However, the advantages associated with repeated synthesis do not eliminate problems such as incomplete reactions, aggregation, sequential deletions, or sequence-dependent side reactions.[4][6][7]

Classical synthesis in solution maintains the peptide intermediates in a liquid phase and can provide greater flexibility in isolating and characterizing the individual intermediates. However, multiple separations and deteriorating solubility can become serious limitations.

Methods of liquid phase synthesis with soluble markers attempt to combine solution chemistry with designed material recovery strategies. They can therefore be an additional option in the case of sequences and production problems for which conventional SPPS is not optimal.[8][9]

Longer or particularly complex molecules can also be prepared by assembling fragments, including native chemical ligation, rather than relying solely on the continuous addition of successive amino acid residues.[10]

Regardless of the method chosen for the synthesis, the efficiency of the coupling and the chemistry of the protecting groups remain crucial, because slight inefficiencies and side reactions can accumulate during subsequent stages of the synthesis. The formation of aspartimide, the delocalization sequences, and the stereochemical changes show why the intended sequence alone does not describe everything that can be formed during the process.[2][11][12]

The work does not end even with the placing of the chain.

Purification determines whether the target product can be separated from other components.

Subsequently, appropriately selected analytical methods address specific questions regarding identity, purity, content, stereochemistry, conformation, and – if relevant – functional activity.[3][12][13]

Increasing the scale of production introduces further issues regarding mass transport, resin retention, solvent consumption, filtration, purification capabilities, process repeatability, and its environmental impact. For this reason, the industrial evaluation of the process should encompass synthesis, purification, and isolation together, rather than focusing solely on the rate of coupling. [14] [15]

The most important principle is the following:

Effective peptide production requires control of the entire process – from the molecular design and bond formation to purification and structural characterization. Obtaining a chain with the expected nominal mass or a high peak size in chromatography represents valuable evidence, but none of these parameters alone can confirm the full molecular identity or the quality of the production process.

Disclaimer

The article is purely educational and scientifically informational in nature. It describes the general principles of peptide chemistry, solid-phase synthesis, solution-phase synthesis, protective group strategies, coupling, purification, analytical characterization, production processes, and examples derived from published studies. It is not a laboratory protocol, manufacturing instruction, medical advice, a guide to dosage or administration, or a quality certificate for any particular peptide product. Experimental results should be interpreted in the context of the sequence, the methods used, the scale of the process, the analytical techniques, and the limitations of individual studies.

References

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