The Design of Amino Hydroxamic Metal Chelates and Their Activity in Biological Systems. Formation Constants between 2-Amino-4-methylpentanehydroxamic Acid with Proton, Copper(II), and Iron(III) in Aqueous Solution

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Abstract
The complex-formation equilibria and relative stability constants of species present in aqueous solution of copper(II) and iron(III) with “l-leucinehydroxamic acid” (2-amino-4-methylpentanehydroxamic acid, ahmpe) have been determined by using potentiometric and spectrophotometric titrations in 0.5 mol dm−3 KCl solution at 25°C. Their acid-base reaction stoicheiometries have been evaluated and the protonation constants calculated from potentiometric data with the aid of the GAUSS Z and SUPERQUAD programs. The complex formation constants have been determined by using SQUAD (absorbance data), SUPERQUAD and STBLTY (potentiometric data) programs. The ligand is bound to the metal ions through the coordination of the N atoms of the α-amino group and the deprotonated –NHO− group in copper(II)–ahmpe system. For iron(III)–ahmpe system the coordination is referred to oxygen atoms of carboxylate and hydroxamate moieties. The UV-visible studies provide important evidence for the formation of several complex species depending on the pH. The experimental curves [ε=f(λ)], deduced from refinement of absorbance data with the program SQUAD, have been split up into precisely positioned absorption bands by Gaussian analysis using a nonlinear least-squares computer program NLIN. So the solution electronic spectra have been also employed to estimate the coordination sphere and have let us to suppose a weak tetragonal coordination [CuL2(H2O)2] and octahedral for Fe3+ complexes. The determination of formation constants and the coordination pattern proposed have a great interest in terms of metal chelate structures and their important role in biological systems.