How research peptides are made and tested: from synthesis to COA

Prepared by: Research Desk. Sources are linked for verification.

Overview

A synthetic research peptide is assembled as a chain, released from a solid support and then analysed. The certificate of analysis (COA) records the test results for one batch. This guide covers chain assembly, by-products, counterions and the main quality-control tests. It ends with a COA checklist and a regulatory reference point. It describes chemistry and documentation only and makes no claims about the uses or effects of any compound.

Passages marked "general context" reflect widely used analytical practice, but the sources gathered for this guide do not include a primary reference for them.

Building the chain: solid-phase synthesis

In 1963 R. B. Merrifield described solid-phase peptide synthesis (SPPS) in the Journal of the American Chemical Society, using it to make a tetrapeptide (bibliographic record). The growing peptide is attached to an insoluble resin, and repeated rounds of deprotection and amino-acid coupling extend the chain. Because the peptide stays on the resin, excess reagents and by-products can be washed away between steps. This approach later made automated peptide production possible.

Fmoc and Boc strategies

General context. Each incoming amino acid carries a temporary protecting group on its N-terminus, removed before the next coupling.

• Boc chemistry removes the N-terminal group with acid, which usually calls for a resin and linker strategy suited to the acid conditions used for final cleavage.
• Fmoc chemistry removes its protecting group under basic conditions, commonly with piperidine, and often relies on acid for final cleavage from the resin.

Neither strategy is better in every case. The choice depends on the sequence, the protecting groups, the resin and the intended product.

Cleavage and by-products

Once assembly is complete, the chain is cleaved from the resin. The crude material contains the target peptide along with related by-products. A review of related impurities in peptide medicines describes the main types: deletion and insertion sequences, incomplete deprotection, oxidation, deamidation and diastereomers (D'Hondt et al., 2014, PubMed).

General context on how some of these arise:

• Deletion and truncated sequences can follow a failed coupling or deprotection step, or early chain termination.
• Epimers can form by racemisation during activation or coupling and share the nominal mass of the intended peptide.
• Aspartimide-derived species come from a sequence- and condition-dependent side reaction at aspartic acid, which can yield isoAsp or epimerised forms.
• Oxidation commonly affects susceptible residues such as methionine, tryptophan or cysteine.

These classes are covered further in synthetic peptide impurities.

Purification, drying and counterions

The sources gathered here do not describe preparative RP-HPLC purification or freeze-drying (lyophilisation) in enough detail to support specific statements, so those steps are only named.

The finished solid is rarely pure peptide by weight. Its gross mass can include water, residual solvent, salts and counterions such as trifluoroacetate (TFA) or acetate, which matters when reading weight-based figures on a COA.

Quality control tests

General context applies to every paragraph in this section.

HPLC purity at 214 or 220 nm. RP-HPLC separates peptide-related species by hydrophobicity. UV detection near 214 nm is common because peptide bonds absorb strongly there; 220 nm is also used. Purity is usually an area percentage: the share of detected UV peak area assigned to the main peak, not the mass fraction of peptide. Co-elution, differing detector response and weakly absorbing impurities can all affect the figure.

Mass spectrometry for identity. ESI-MS or MALDI-MS can confirm that the main component has the expected mass. MS supports identity, not purity, and a matching mass does not prove sequence order or distinguish every isomer.

Net peptide content. This estimates how much of the weighed solid is peptide, after accounting for water, salts and counterions, and needs a separate test such as amino acid analysis. A batch can show high HPLC purity but lower peptide content, as explained in HPLC purity vs peptide content.

Water and residual solvents. Reported separately where relevant.

Endotoxin. Bacterial endotoxin is generally assessed with a test based on Limulus amebocyte lysate (LAL). A meaningful result states units and the applicable limit, and method suitability and interference controls matter for the sample matrix. No compendial procedure or limit is cited here.

A pharmaceutical reference point: FDA 2021 guidance

In May 2021 the US Food and Drug Administration (FDA) issued guidance on abbreviated new drug applications (ANDAs) for certain highly purified synthetic peptides (guidance text), also summarised in an FDA presentation and summary. Its scope is narrow: generic versions of glucagon, liraglutide, nesiritide, teriparatide and teduglutide, where the reference listed drug is of recombinant-DNA origin. For those products, the guidance:

• generally recommends identifying peptide-related impurities at 0.10% or more of the drug substance;
• does not accept a new specified impurity above 0.5% for this approach;
• expects new impurities from 0.10% to 0.5% to be characterised and justified, including consideration of safety, effectiveness and immunogenicity risk;
• expects any impurity also found in the reference listed drug to be no higher than in that product.

These thresholds are a benchmark for five named products, not a general specification for research-grade peptides or a standalone release limit.

What a COA should contain

ICH Q7 (which defines a batch) and ICH Q2(R2) (on validating analytical procedures) are listed on the ICH quality guidelines page. Some laboratories are accredited to ISO/IEC 17025 for a defined scope of tests.

Limits of a COA

A COA describes only the sample tested and only the tests listed; a report showing only HPLC and MS says nothing about water, counterion content or endotoxin. It is an analytical record, not evidence of suitability for any purpose, and a research-use COA is not automatically equivalent to a pharmaceutical drug-substance specification.

Sources

• Merrifield, "Solid Phase Peptide Synthesis. I. The Synthesis of a Tetrapeptide", JACS (1963): bibliographic record
• D'Hondt et al., 2014, review of related impurities in peptide medicines: PubMed
• FDA, ANDA guidance for certain highly purified synthetic peptide drug products (2021): guidance text
• FDA presentation on the 2021 guidance: fda.gov
• FDA summary of synthetic peptide guidance: fda.gov
• ICH quality guidelines
• ISO/IEC 17025

Supplier documentation

Purita is a store that sells peptides for laboratory research use only. It publishes a certificates page where reports are available for some products; reports are not yet available for every item, so check the page for the specific material.

Visit the Purita website

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Related pages

• Peptide design, synthesis and testing, explained
• About DesignPeptide
• HPLC purity vs net peptide content: two numbers that are often confused
• Common impurities in synthetic peptides and how labs detect them