Peptide terminology explained for technical buyers

Sequence, residue and modification

A peptide is defined by its amino acid sequence written from the N-terminus to the C-terminus. Residue positions matter because a single substitution creates a different molecule with a different CAS number. Common modifications include N-terminal acetylation, C-terminal amidation, cyclisation, disulfide bridging, PEGylation and fatty acid acylation, each of which changes mass, solubility and stability.

Purity versus peptide content

Chromatographic purity, usually by reverse-phase HPLC at a stated wavelength, describes the proportion of peak area attributable to the target. Peptide content, determined by nitrogen analysis or amino acid analysis, describes how much of the physical powder is peptide at all. The remainder is typically water, counter-ion such as acetate or trifluoroacetate, and residual salts. A material can be 99 percent pure by HPLC and well under 80 percent peptide by mass.

Net peptide and dosing arithmetic

Net peptide is purity multiplied by peptide content. Any quantitative work must use net peptide, not the gross weight of the vial, and the supplier should state which basis a quoted quantity refers to.

Counter-ion and residual solvent

Trifluoroacetate from purification and acetate from ion exchange behave differently in downstream use. Ask which counter-ion is present and at what level, and ask for residual solvent data.

Lyophilised form and reconstitution

Lyophilised peptide is a cake whose appearance says little about quality. Handling terms worth confirming are storage temperature, protection from light and moisture, container closure, and whether stability data exist for the storage condition claimed.

Identity confirmation

Mass spectrometry confirms molecular mass; it does not by itself confirm sequence order. Where sequence matters, ask for peptide mapping or sequencing data rather than mass alone.

Grade language

Research use only is a statement about permitted use, not a quality standard. It never implies suitability for human or animal administration.