Definition
A single amino acid unit within a peptide chain, after it has lost water in forming peptide bonds.
In peptide chemistry, a residue is one amino acid as it exists within a chain. When amino acids join through peptide bonds, each loses the elements of water, so a residue's mass is the free amino acid's mass minus about 18 Da. Peptide length is described in residues: a 10-residue peptide is a decapeptide, and GHRH has 44. Residues are numbered from the N-terminus, so notations such as Aib8 or GHRH(1–29) refer to specific positions or ranges. Residue masses summed, plus one water for the termini, give the theoretical molecular weight used for mass spectrometry. Certain residues carry known liabilities: methionine and tryptophan oxidise, asparagine deamidates and aspartate can isomerise.