Thermodynamics of formation of binuclear complexes of some sulphur-containing dipeptides with silver(I) and copper(II). Crystal structure of a methionyl-S-methylcysteine complex of copper(II)

Abstract

Formation constants are reported at 25 °C and I= 0.10 mol dm^–3(KNO₃) for binuclear complexes of nine sulphur-containing dipeptides with Ag^I and Cu^II, together with the formation constants of the parent Cu^II complexes. Dipeptides (HL) studied were Gly-L-Met, L-Met-Gly, L-Met-L-Met, L-Met-D-Met, Cys(Me)-L-Met, Cys(Me)-D-Met, L-Met-Cys(Me), D-Met-Cys(Me), and Cys(Me)-Cys(Me)[Met = methionine, Cys(Me)=S-methyl-L-cysteine]. The major ternary complex with dipeptides of two sulphur-containing amino acids of the same chirality was [AgCuH_–1L]⁺. When the amino-acid residues were of opposite chirality, ternary complexes were less stable by a factor of over 20, giving very dramatic stereoselectivity. The crystal structure of [CuH_–1{L-Met-Cys(Me)O}][L-Met-Cys(MeO)=L-methionyl-S-methylcysteinate(1–)] is reported. The copper atom is five-co-ordinate (square pyramidal) with no Cu–S interaction.