Polypeptide
Definition
A chain long enough that counting residues stops being useful, conventionally above about fifty, at which point most writers switch to calling it a protein.
What this means for research sourcing
Structure and identity terms are what a certificate of analysis is written in. Every Volta compound ships with its sequence, molecular weight and measured purity, so these terms map directly onto the specification on the product page.
Related Terms in Chemistry & Structure
Dipeptide
Two residues joined by a single peptide bond. Di- and tripeptides are the only chain lengths the intestinal PepT1 transporter carries intact, which is why they come up in discussions of oral absorption.
Tripeptide
Three residues joined by two peptide bonds. GHK, the copper-binding tripeptide, is the example most often met in this vertical.
Tetrapeptide
Four residues. Epithalon (Ala-Glu-Asp-Gly) and several of the bioregulators are tetrapeptides.
Pentapeptide
Five residues. The prefix is a residue count and nothing more, which is worth saying because cosmetic ingredient names lean on it heavily.
Pentadecapeptide
Fifteen residues. BPC-157 is routinely described this way in the literature, so the word is more common in peptide writing than its length would suggest.
Disulfide Bond
A covalent bridge between the sulfur atoms of two cysteine residues. It staples distant parts of a chain together and holds a fold in place, and reducing conditions break it, which is one way a peptide loses activity without losing mass.
Cysteine
The amino acid whose thiol side chain forms disulfide bridges. It is also one of the three residues most readily oxidised, so a sequence containing it has a predictable degradation liability.
Peptide Fragment
A section cut from a longer peptide or protein and studied on the assumption that it carries part of the parent's activity. The assumption is not automatic: a fragment can retain binding and lose function, or do neither. HGH Fragment 176-191 is named for the residues it spans.