Copper(II) Complexes of Diastereoisomeric Methionylmethionines in Aqueous Solution. Favoring of the Amide-Deprotonated Complex in the L,L-Dipeptide without Sulfur Coordination

A thermodynamic investigation has been carried out on copper(II) complexes with L-methionyl-L-methionine or D-methionyl-L-methionine at 25 °C and I = 0.1 mol dm-3(KNO3). Thermodynamic stereoselectivity has been found in the formation of the amide-deprotonated complexes. In particular, the formation of the copper(II) complex with the L,L-dipeptide is enthalpically favored. Structural information obtained by the EPR spectra in solution parallels the thermodynamic data: a slightly larger copper hyperfine coupling constant has been determined in the case of the copper(II) complex with L-methionyl-L-methionine. Comparison with data previously obtained for similar dipeptides containing noncoordinating side-chain groups evidences that the sulfur atom is not coordinated to the metal ion. The thermodynamic stereoselectivity of the amide-deprotonated complex differences of 0.3 in log β and 1.7 kcal mol-1in ΔH° can be attributed to the hydrophobic interaction between the residues of side chains. This noncovalent “bonding” is possible only for the L,L-diastereoisomer where the side chains are on the same side of the coordination plane.