A Thermodynamic and Spectroscopic Study of the Proton and Copper(II) Complexes of L-Prolyl-L-histidine, D-Prolyl-L-histidine, L-Histidyl-L-histidine, and D-Histidyl-L-histidine.

The syntheses of optically pure L-Pro-L-His, D-Pro-L-His, L-His-L-His, and D-His-L-His (HL)(Pro = proline, His = histidine) are reported together with the results of a potentiometric and spectroscopic study of their proton and copper(II) complexes at 25 °C and I= 0.10 mol dm–3(KNO3). While stereoselectivity is present between both pairs of diastereoisomers, the co-ordination sequences are similar. The Pro-His isomers form the series of complexes [CuL], [CuH–1L], [CuH–2L], and [CuH–3L](charges omitted), His-His isomers form the series [Cu(H2L)], [Cu(HL)], [CuL], [Cu2H–1L2], and [(CuH–1L)2] probably returning to the monomer [CuH–2L] above pH 10. Both the His-His diastereoisomers form dimers above pH 6.5 with each Cu2+ ion co-ordinated to four nitrogen donors. The stereoselectivity in both proton and copper(II) complexes may be explained by considering the preferred trans conformations of the peptide chains.