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Peptide Science Glossary

A reference for the vocabulary that peptide literature, certificates of analysis and regulatory documents assume you already have.

RetraLabs Scientific Content TeamReviewed by RetraLabs Laboratory Quality Review3 min readUpdated 11 October 2026

Structure and chemistry

The vocabulary of what a peptide is and how it is built.

Amide bond — the covalent link between amino acids; also called a peptide bond.
Residue — one amino acid within a chain, after the water lost in bond formation.
N-terminus / C-terminus — the free amino and carboxyl ends; sequences are written N to C.
Primary sequence — the order of residues, conventionally in one-letter or three-letter code.
Disulfide bridge — a covalent bond between two cysteine residues, constraining conformation.
Cyclic peptide — a chain closed into a ring, usually more protease-resistant than its linear form.
D-amino acid — the mirror-image form of the natural L configuration; resists most proteases.
Retro-inverso — a reversed sequence built from D-amino acids, approximating the parent side-chain topology.
Lyophilisation — freeze-drying, which removes water and so slows hydrolysis.
Counter-ion — the ion accompanying a charged peptide, commonly acetate or trifluoroacetate.

The vocabulary of a certificate of analysis.

RP-HPLC — reversed-phase high-performance liquid chromatography; separates by hydrophobicity.
Area percent purity — the target peak's share of total detected peak area; only counts what the detector sees.
Orthogonal method — a second method with different selectivity, used to expose co-elution.
Deletion sequence — a synthesis by-product missing one residue; structurally close to the target.
ESI / MALDI — two ionisation techniques; ESI gives multiply charged ions and couples to LC.
Monoisotopic mass — calculated using the lightest isotope of each element.
Tandem MS (MS/MS) — fragmentation of a selected ion, allowing sequence to be read.
Peptide mapping — enzymatic digestion followed by analysis of the fragments, for identity.
Amino-acid analysis (AAA) — hydrolysis and quantitation; gives peptide content by mass.
Karl Fischer titration — quantifies water content.
Peptide content — the fraction of a powder's mass that is peptide, as distinct from purity.
Reference standard — a characterised material with assigned values and stated uncertainty.
Internal standard — a compound added in known amount to correct for losses and variability.

The vocabulary of what compounds do.

Agonist / antagonist — activates a receptor / blocks activation by others.
GPCR — G-protein-coupled receptor; most peptide hormone receptors are of this family.
Half-life — time for circulating concentration to fall by half.
DPP-4 — dipeptidyl peptidase-4, the enzyme that rapidly cleaves native GLP-1.
Albumin binding — reversible association with serum albumin, used to extend duration.
Receptor downregulation — reduced receptor availability after sustained stimulation.
Surrogate endpoint — a measured marker standing in for a clinical outcome.

The vocabulary of status and proof.

Research use only (RUO) — supplied for laboratory use; not assessed as a medicine.
Investigational — studied in human trials; not approved.
Preclinical — laboratory and animal work, before human studies.
Marketing authorisation — a regulator's approval to sell a medicine for an indication.
CDSCO — India's Central Drugs Standard Control Organisation.
ISO/IEC 17025 — accreditation standard for testing laboratory competence.
ISO 17034 — standard for reference material producers.
GMP — good manufacturing practice; a pharmaceutical quality system, not applicable to RUO material.