endently represent an oxo or imino function; and wherein furthermore:R13 is a hydrogen atom, a C1 -C4 alkyl group or a carboxy(C1 -C4)alkyl group; R14 is a hydrogen atom or an amino group; n is 0, 1 or 2; A represents an amino group or a mercapto group; and B represents a mercapto group, or a group of the general formula NH--Z--CO--NH--m Z--COOH, wherein:Z is an optionally substituted C1 -C4 alkyl