Peptide science basics

A peptide is a chain of amino acids joined by amide bonds between the carboxyl group of one residue and the amino group of the next. The boundary with proteins is a convention rather than a chemical distinction: chains up to roughly fifty residues are generally called peptides, longer ones proteins. The compounds in this catalog sit well below that line, in the range of five to forty residues.

Identity is the amino acid sequence, read from the N-terminus to the C-terminus, together with any modifications and the stereochemistry at each center. Change one residue and the compound is a different one, whatever the label says. This is why the analytical question for a supplier is never “is this a peptide” but “is this the sequence claimed, and what else is in the vial”.

Size is quoted as molecular weight, in daltons. It is calculated from the molecular formula, so it is a derived figure rather than a measured one, and it is what mass spectrometry checks a sample against.

Residues are written left to right from the N-terminus using either one-letter or three-letter codes. Modifications are appended in brackets or as prefixes and suffixes: an amidated C-terminus is written -NH2, an acetylated N-terminus Ac-, and D-amino acids are marked explicitly because the L form is assumed by default.

Those annotations are not decoration. An amidated C-terminus removes a negative charge and changes the molecular weight by one dalton relative to the free acid, which is enough to change how a mass spectrum is interpreted. A specification that omits terminal modifications is ambiguous about which compound it describes.

Research peptides in this size range are made by solid-phase peptide synthesis. The C-terminal residue is anchored to an insoluble resin bead, and the chain is extended one residue at a time in a repeating cycle: deprotect the terminal amine, couple the next activated amino acid, wash away the excess. Because the growing chain stays attached to the resin, purification between steps is filtration rather than chromatography, which is what makes the process practical at all.

Yield compounds. Each coupling step runs at high but not perfect efficiency, and the shortfall accumulates: at 99% per step a thirty-residue peptide retains about 74% of the theoretical yield, and at 98% about 55%. The missing material is not absent, it is present as deletion sequences that are one residue short. Those by-products closely resemble the target, which is exactly why they are the impurities hardest to separate and the ones an orthogonal identity method exists to catch.

After assembly the chain is cleaved from the resin and the side-chain protecting groups are removed, usually with a trifluoroacetic acid cocktail. The crude material is then purified by preparative reversed-phase chromatography, collected as fractions, and the fractions meeting specification are pooled.

The pooled fractions are frozen and dried by lyophilization, which removes water by sublimation under vacuum and leaves a porous cake or a thin film on the glass. A dry solid is chosen because hydrolysis, oxidation and deamidation all proceed far faster in solution.

The solid is not pure peptide by mass, and no honest supplier claims it is. It also contains the counterion / salt form carried through from purification, residual water that survived secondary drying, and traces of the chromatography solvent. The fraction that is peptide is the net peptide content, and it is a separate measurement from purity.

Appearance tells you very little. A collapsed cake indicates the formulation warmed above its collapse temperature during drying, which is a process observation rather than a purity result, and a few milligrams of peptide may leave a film barely visible against the glass. Neither observation substitutes for an assay, and neither is grounds for a conclusion about the material.

degradation in the dry state is slow but not absent. The routes are hydrolysis of the backbone, oxidation at methionine, cysteine and tryptophan residues, deamidation of asparagine and glutamine, and aggregation. Each is accelerated by some combination of temperature, moisture, light and dissolved oxygen, which is why storage is specified in terms of all four rather than temperature alone.

None of this is visible. A vial that looks unchanged can have lost several percent of intact compound, and the only way to know is to re-assay it. The practical consequence is that storage conditions are part of the product specification rather than a shipping courtesy.