A peptide's fragility is written in its sequence. Methionine, cysteine and tryptophan oxidize; asparagine and glutamine lose their amide groups; the bond after an aspartate hydrolyses in acid and rearranges above neutral pH. Freezing and thawing a solution adds physical stress on top of that chemistry, which is why manufacturers' guides count repeated freezing as a stress in its own right. Below, the reactions are mapped onto the sequences in the catalog (how to read a peptide sequence explains the one-letter codes); this is chemistry, and the site publishes no use instructions.

Which residues are fragile, and what happens to them

Bachem's handling guideline singles out five residues: "Peptides containing Asn, Gln, Met, Cys, and/or Trp have limited shelf lives." GenScript's guide names the same five plus aspartate and an N-terminal glutamate.

Two details matter for reading a certificate. First, the oxidation products are heavier than the parent by a whole oxygen atom, so a methionine sulfoxide shows up in mass spectrometry as a +16 Da species; a 2018 review in Antioxidants puts it plainly: "Methionine oxidation, by the addition of an extra oxygen atom, leads to the generation of methionine sulfoxide." The same review describes cysteine's path as oxidation "first" to "either the formation of a disulfide bridge or a sulfenic acid."

Second, deamidation is pH-switched. In the model hexapeptide Val-Tyr-Pro-Asn-Gly-Ala, a fragment of ACTH, Patel and Borchardt found that "in the pH range 5 to 12, the peptide deamidated exclusively via a cyclic imide intermediate with the formation of both the Asp- and the isoAsp-hexapeptides," whereas "at acidic pH's, the pathway of deamidation involved direct hydrolysis of the amide side chain of Asn residue to produce only the Asp-hexapeptide." Isoaspartate is the awkward product: same mass as aspartate, different backbone, and a different retention time on a good column.

Aspartate has its own two-way chemistry. Using the matching Val-Tyr-Pro-Asp-Gly-Ala peptide, Oliyai and Borchardt showed that at very acidic pH the peptide "predominantly underwent specific acid-catalyzed hydrolysis of the Asp-Gly amide bond," while "at pH values above 6.0" the only product was the isoaspartate isomer. The Asp-Gly pair is the textbook weak point, because glycine's lack of a side chain leaves the backbone free to form the cyclic imide.

Tryptophan's sensitivity was demonstrated in antibody work rather than in short peptides, but the chemistry is the same indole ring: Hensel and colleagues identified "a solvent accessible tryptophan residue ... susceptible to oxidation under real-time storage and elevated temperature conditions," and found it oxidized to a greater degree than the antibody's known methionine sites.

Hygroscopic residues are a separate problem

Water uptake is not a chemical reaction, but it feeds one. GenScript's guide states that "peptides containing Asp, Glu, Lys, Arg, or His are prone to moisture absorption from the air, called deliquescence." A powder that has taken up water is partly a solution, and every reaction in the table above runs faster in solution than in a dry solid. Bachem's care note adds that moisture also changes the arithmetic of the vial: the peptide content of a solid "may range from 70 to 90% as peptides contain counter ions (e.g. acetate, trifluoroacetate) and residual moisture." That is the subject of net peptide content.

What freezing and thawing do physically

The chemistry above needs water and molecular mobility. Freezing removes both; that is the premise of every manufacturer's frozen-storage guidance. The trouble is the transition. Studies on protein therapeutics, which are the best-characterised case, describe four stresses that act during every freeze and every thaw. Short peptides have far less three-dimensional fold to lose than a protein, but the concentration and pH effects apply to any solute.

• Freeze concentration. Ice crystals are pure water, so everything else is pushed into the shrinking liquid between them. Wöll and Hubbuch describe "freeze concentration of all solutes (e.g. buffer components, excipients, protein) due to ice crystal formation." Salt and peptide concentrations in that liquid can be many times the nominal value.
• The ice surface. The same authors list "the growing ice surface, due to the possible denaturation of the protein molecules on this surface" as a stress in its own right. Molecules adsorb to the ice-liquid boundary and can unfold there.
• pH shift. Buffer salts do not all crystallize at once. Jain and colleagues note that "as a solution freezes, the pH of a phosphate buffered solution is expected to drift due to the differential solubility of the phosphate salts and the selective precipitation of the disodium phosphate." A peptide frozen at pH 7 can sit in a much more acidic pocket while the solution is partly frozen, which is exactly the condition that favours Asp-X cleavage.
• Cold unfolding and slow thaw. Jain's group also observed that "slow thawing is contributing to aggregate formation," because longer thaw times give unfolded molecules more time to find each other. Which sequences aggregate most readily is covered in why some peptides dissolve badly.

None of this happens in a lyophilized cake, which is why the dry solid is the reference format for shipping and long storage (what lyophilized means).

What manufacturers' guides say about repeated freezing

Bachem's guideline describes a peptide in solution as less stable than the dry solid and describes division into single-use portions as its practice for limiting repeated freezing. The same guideline identifies peptides carrying Asn, Gln, Cys, Met or Trp as the least stable in solution. GenScript's guide identifies repeated freezing and thawing as a degradation stress for both the solid and solutions.

On the mechanism, the same guide notes that aliquoting "reduces the amount of air exposure to the peptide."

The logic combines the two sections above. Each freeze and thaw is one more pass through concentration, ice-interface and pH stresses, and each opening of a container is one more exposure to oxygen and humidity. A set of portions that are each frozen once experiences those stresses once. That is a statement about the molecule, and it is the whole of what the manufacturers claim. It is also a different claim from the "28 days" figure that circulates online, which has a different origin altogether (where the 28 days number comes from).

Where these residues sit in the sequences we stock

The table applies the chemistry to sequences published on our product pages. It says what a residue can do, not what any lot has done; lot-specific purity and identity data live on each certificate.

The last row shows design against oxidation: norleucine is methionine with a methylene group where the sulfur would be, so the residue keeps its shape and loses the site that forms a sulfoxide.

How degradation products appear on a certificate

Oxidized and deamidated species are the parent peptide plus or minus a few daltons, seen as small peaks near the main peak on a reversed-phase chromatogram: a methionine sulfoxide runs earlier than its parent on most columns and appears at +16 Da, and a deamidated asparagine appears at about +1 Da and may split into aspartate and isoaspartate peaks. A single purity number cannot say which of these is present; the chromatogram and mass list can, and reading a chromatogram shows what those neighbouring peaks look like.

Sources

• Handling and Storage Guidelines for Peptides, Bachem, undated
• Care and Handling of Peptides, Bachem technical note, undated
• Peptide Storage and Handling Guidelines, GenScript, undated
• Custom Peptide Technical Resource, GenScript, undated (PDF)
• Patel K, Borchardt RT. Chemical pathways of peptide degradation. II. Kinetics of deamidation of an asparaginyl residue in a model hexapeptide. Pharm Res, 1990
• Oliyai C, Borchardt RT. Chemical pathways of peptide degradation. IV. Pathways, kinetics, and mechanism of degradation of an aspartyl residue in a model hexapeptide. Pharm Res, 1993
• Lourenço dos Santos S, Petropoulos I, Friguet B. The Oxidized Protein Repair Enzymes Methionine Sulfoxide Reductases. Antioxidants, 2018
• Hensel M et al. Identification of potential sites for tryptophan oxidation in recombinant antibodies. PLoS One, 2011
• Jain K, Salamat-Miller N, Taylor K. Freeze-thaw characterization process to minimize aggregation and enable drug product manufacturing of protein based therapeutics. Sci Rep, 2021
• Wöll AK, Hubbuch J. Investigation of the reversibility of freeze/thaw stress-induced protein instability. Bioprocess Biosyst Eng, 2020
• Mateescu DM et al. BPC-157 as an Investigational Peptide Therapeutic: Biopharmaceutical Challenges, Formulation Strategies. Pharmaceutics, 2026
• Pro-opiomelanocortin, POMC, human (P01189), UniProt