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⁺, Co^II, Ni^II, and Cu^II 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 β_pqr=[M_pH_qL_r]/[M]^p[H]^q[L]^r were obtained: hasn, log β₀₁₁= 9.37(1), log β₀₂₁= 17.52(1), log β₀₃₁= 19.70(1); Co^II–hasn, log β₁₁₁= 13.91(2), log β₁₀₁= 7.56(2), log β₂₀₃= 23.43(3), log β₁₀₂= 12.86(3), log β_1–12= 2.46(7); Ni^II–hasn, log β₁₁₁= 14.25(3), log β₁₀₁= 8.38(3), log β₁₀₂= 14.73(6), log β₂₀₃= 25.78(4); Cu^II–hasn, log β₁₀₁= 12.60(1), log β₂₀₃= 37.45(5), log β₃₀₄= 53.43(9), log β₁₀₂= 19.07(4), log β_1–12= 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.