Metal ion binding capability of secondary (N-methyl) versus primary (N-H) dipeptide hydroxamic acids

Novel, dipeptide hydroxamic acids, L-Ala-L-AlaN(Me)OH and L-Ala-L-SerN(Me)OH, as well as the corresponding Z-protected ones, Z-L-Ala-L-AlaN(Me)OH and Z-L-Ala-L-SerN(Me)OH (Z = benzyloxycarbonyl) were synthesized and the metal ion binding capabilities toward Fe(III), Al(III), Ni(II), Cu(II) and Zn(II) were studied by combined pH-potentiometric and spectroscopic (UV-VIS, EPR) methods in aqueous solution. Each of these derivatives contains a methyl substituent at the hydroxamate N making it unsuitable for metal ion coordination. By making a comparison of the obtained results with our previously published ones for the corresponding primary hydroxamates, the effects of the replacement of hydroxamic-NH (primary) by hydroxamic-N(Me) (secondary) and that of the protection at the N terminus of the peptide chain (Z-protected ligands) on the metal binding capability as well as metal ion selectivity are evaluated in this paper. Except the significantly decreased solubility of their complexes, the Z-protected derivatives can coordinate as the simple analogous monohydroxamates. Fe(III) and Al(III) with hard character were found to form [O, O] chelated hydroxamate species with all the ligands studied. Like the corresponding primary ligands secondary derivatives were found to form [O, O] and [NH2, CO] chelated linkage isomers with Zn(II). For Ni(II) the deprotonation and coordination of the amide group is disfavoured when the hydroxamate N as binding site is blocked. Unlike primary dipeptide hydroxamates, the secondary ones, in their most stable complex, coordinate to Cu(II) via an [NH2, Namide, Ohydrox.] chelate and the formation of oligonuclear complexes, as it was shown previously with the corresponding primary analogues, cannot be detected. No indication was found for the role of the hydroxyl group of the serine moiety in metal ion binding. The results show that, unlike for the M(III) metal ions, in the case of M(II) ones there is a preference of complexation with the primary derivatives over the secondary ligands. This preference was found to be the most pronounced for Cu(II).