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

Abstract

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.