Possible biological activity in proton, metal, and aminohydroxamic acid equilibria. Protonation and complex formation reactions between N-hydroxy-D-asparagine and cobalt(II), nickel(II), copper(II), and hydrogen ions in aqueous solution

Abstract

The protonation and complex-formation equilibria in aqueous solutions between H+, CoII, NiII, and CuII and N-hydroxy-D-asparagine (hasn) have been investigated by the potentiometric method in 0.5 mol dm–3 KCl solution at 25 °C. The specific tendencies of aminohydroxamic acids towards different metal ions have been tested as models for substratum–metal bonding in biological reactions. The following overall formation constants βpqr=[MpHqLr]/[M]p[H]q[L]r were obtained: hasn, log β011= 9.37(1), log β021= 17.52(1), log β031= 19.70(1); CoII–hasn, log β111= 13.91(2), log β101= 7.56(2), log β203= 23.43(3), log β102= 12.86(3), log β1–12= 2.46(7); NiII–hasn, log β111= 14.25(3), log β101= 8.38(3), log β102= 14.73(6), log β203= 25.78(4); CuII–hasn, log β101= 12.60(1), log β203= 37.45(5), log β304= 53.43(9), log β102= 19.07(4), log β1–12= 9.08(4). The ligands are bound to the metals through the N atom of the α-amino group, and by the deprotonated NHO– group via the N or O atom. Electronic spectra under the same experimental conditions have been recorded and discussed. They provide important evidence for the formation of different metal complexes of N-hydroxy-D-asparagine, depending on the pH and can also be used to estimate the co-ordination sphere around the metal ions and to observe the equilibria between different complexes.