When two or more amino acids combine to form a @[email protected], 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 @[email protected] 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 @[email protected], 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.
White Book, 2nd ed., p. 48 [Terms] [Book]