Quaternary Structure
Definition
The arrangement of two or more folded chains into one functional assembly. It is the level above tertiary structure and applies only to multi-subunit molecules, which most research peptides are not.
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
Protein Folding
The process by which a chain adopts its three-dimensional shape, driven mainly by burying hydrophobic residues away from water. Activity usually depends on the fold, which is why a denatured molecule can be chemically intact and inert.
Glycosylation
Attachment of sugar chains to a protein, typically at asparagine or at serine and threonine. It affects stability, solubility and clearance, and it is a reason a recombinant molecule may differ from the same sequence made synthetically.
Post-Translational Modification
Any chemical change made to a chain after it is assembled: acetylation, amidation, phosphorylation, glycosylation and others. The same sequence with and without one is two different molecules with two different masses.
Residue
One amino acid once it sits inside a chain, so called because the condensation that formed the bond left part of the free molecule behind. A peptide described as fifteen residues long has fifteen amino acids in it.
N-Terminus and C-Terminus
The two ends of a peptide: the N-terminus carries the free amino group, the C-terminus the free carboxyl group. Sequences are written N to C by convention, and end modifications such as acetylation or amidation are named for whichever terminus they cap.
Oligopeptide
A short chain, usually taken to mean somewhere between two and twenty residues. The word is descriptive rather than functional and says nothing about what the molecule does.
Polypeptide
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.
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.