When two or more amino acids combine to form a peptide
, the elements of water are removed, and what remains of each amino acid is called
an amino-acid residue. α-Amino-acid residues are therefore structures that lack a
hydrogen atom of the amino group
), or the hydroxyl moiety
of the carboxyl group
), or both
); all units of a peptide chain are therefore amino-acid residues. (Residues of amino
acids that contain two amino groups or two carboxyl groups may be joined by isopeptide bonds
, and so may not have the formulas shown.) The residue in a peptide that has an amino
group that is free, or at least not acylated by another amino-acid residue (it may,
for example, be acylated or formylated), is called N-terminal; it is at the N-terminus.
The residue that has a free carboxyl group, or at least does not acylate another amino-acid
residue, (it may, for example, acylate ammonia to give
), is called C-terminal.
IUPAC. Compendium of Chemical Terminology, 2nd ed. (the "Gold Book"). Compiled by
A. D. McNaught and A. Wilkinson. Blackwell Scientific Publications, Oxford (1997).
XML on-line corrected version: http://goldbook.iupac.org (2006-) created by M. Nic,
J. Jirat, B. Kosata; updates compiled by A. Jenkins. ISBN 0-9678550-9-8. https://doi.org/10.1351/goldbook