CHARACTERIZATION OF AMINO HYDROXAMIC METAL CHELATES AND THEIR ACTIVITY IN BIOLOGICAL SYSTEMS. FORMATION CONSTANTS BETWEEN 2-AMINO-N, 3-DIHYDROXYPROPANAMIDE AND 2-AMINO-N, 3-DIHYDROXYBUTANAMIDE AND IRON(III) IONS IN AQUEOUS SOLUTION

Abstract

Complex formation equilibria and relative stability constants for species present in aqueous solutions of ion(III) with “L-serinehydroxamic acid” (2-amino-N,3-dihydroxypropanamide, adhp) and “DL-threoninehydroxamic acid” (2-amino-N,3-dihydroxybutanamide, adhb) have been investigated by potentiometric titrations at 25 ± 0.1°C and I = 0.5 mol dm⁻³ (KCl). Overall formation constants of several metal complexes were computed from potentiometric data with the hyperquad program. The following cumulative association constants (relative standard deviation values are given in parentheses) β _pqr = [M_pH_qL_r]/[M]^p [H]^q [L]^r were obtained: adhp, log β₀₁₁ = 12.72(1), log β₀₂₁ = 21.62(1), log β₀₃₁ = 28.38(1), log β₁₂₂ = 32.88(6), log β₁₁₁ = 17.07(6), log β₁₁₂ = 28.80(6), log β₁₀₂ = 22.15(8), log β₁₂₃ = 41.17(7), log β₁₃₃ = 46.26(13), log β₁₁₃ = 34.83(7), log β₁₀₃ = 28.06(7), log β_2–22 = 22.51(12); adhb, log β₀₁₁ = 12.75(1), log β₀₂₁ = 21.61(1), log β₀₃₁ = 28.39(1), log β₁₂₂ = 32.73(3), log β₁₁₁ = 16.90(4), log β₁₁₂ = 28.55(3), log β₁₀₂ = 21.37(7), log β₁₂₃ = 41.16(3), log β₁₃₃ = 46.03(7), log β₁₁₃ = 34.89(3), log β₁₀₃ = 27.76(3), log β_2–22 = 22.34(7), log β_1–13 = 19.23(5). Since the two ligands show behaviour very similar to that of already investigated aminohydroxamic acids without the OH⁻ group, it is suggested that the proton residing on the alcoholic group is not ionized when the complex formation takes place. Consequently, in the refinement process of the complex species, the H₂L⁺ species with only two dissociable groups ([sbnd] NHOH, [sbnd] NH₃ ⁺) has been considered, taking into account log β₀₁₁ and log β₀₂₁ for the two ligands, respectively. Equilibrium constants for their formation and the probable structure of the chelated compounds formed in aqueous solution are the object of discussion in terms of possible significance to biological reactions. Stability is compared with that of analogous chelated compounds.

Key Words:

• aminohydroxamic acids
• protonation
• complex formation
• stability constants
• iron(III) complexes
• potentiometry