Pattern of transition-metal chelates with biological activity. Complex formation equilibria between α-amino-N-hydroxy-1H-imidazole-4-propanamide and cobalt(II), nickel(II), copper(II), and hydrogen lons in aqueous solution

Abstract

The complex-formation equilibria between Co^II, Ni^II, and Cu^II and α-amino-N-hydroxy-1H-imidazole-4-propanamide (ahip) have been investigated by potentiometric titrations in 0.5 mol dm^–3 KCl solution at 25°C. Their acid–base and metal–ligand reaction stoicheiometrics have been determined, and the equilibrium constants of the corresponding proton and metal ion association constants have been calculated from potentiometric data with the aid of the programs BETAREF and SUPERQUAD. The parameters relating to potentiometric calibration curves and the equilibrium chemical models are selected on the basis of a critical evaluation of the least-squares results and of a statistical analysis of the weighted residuals. Protonated and hydrolysed complexes were formed in the pH range 3.0–11.0. The following cumulative formation constants β_pqr=[M_pH_qL_r]/[M]^p[H]^q[L]^r were obtained: ahip, log β₀₁₁= 8.942(2), log β₀₂₁= 16.009(2), log β₀₃₁= 21.400(4); Co^II–ahip, log β₁₁₁= 14.036(23), log β₁₂₂= 26.633(45), log β₁₁₂= 20.861(30), log β₁₀₂= 12.784(50); Ni^II–ahip, log β₁₁₁= 15.009(7), log β₁₂₂= 28.821(17), log β₁₁₂= 22.793(12), log β₁₀₁= 9.27(7), log β₁₀₂= 15.644(13), log β_{1 – 12}= 4.961(17); Cu^II–ahip, log β₁₁₁= 16.916(12), log β₁₂₂= 29.15(9), log β₁₁₂= 22.08(9), log β₁₀₁= 10.698(38), log β₁₀₂= 13.73(8), log β_{1 – 12}= 3.34(8). The probable structures of the chelated compounds formed in aqueous solution were deduced and their protonation and stability constants compared with those of analogous co-ordination compounds. The ligand is bound to the metal ion via N(3) of the imidazole ring, the α-amino nitrogen, and the hydroxyl oxygen of the deprotonated NHO^– group. Equilibrium constants for their formation and visible spectra of the Cu²⁺–ahip system are discussed in terms of the metal chelates structures and their possible significance to biological systems.