Formation equilibria of ternary complexes of copper(II) with (S)-tryptophanhydroxamic acid and both D- and L-amino acids in aqueous solution

Binary copper(II)/(S)-tryptophanhydroxamic acid and ternary copper(II)/(S)-tryptophanhydroxamic acid/D- and L-amino acid systems (amino acid, HA = Phe, Pro, Trp) have been studied both by potentiometry and Vis-spectrophotometry. (S)-tryptophanhydroxamic acid ((S)-2-Amino-3-(indol-3-yl) -N-hydroxypropanamide, HL) forms four binary species ([CuL](+), [Cu2L2H-1](+), [CuL2], [CuL2H-1](-)) in the pH range 3-11, but precipitation of the dinuclear complex occurs between pH 4 and 6. In order to overcome these difficulties, EDTA was used as a competing ligand in order to reduce the concentration of the insoluble species. In the case of ternary systems the concentration of [Cu2L2H-1](+) was lowered by using high amino acid/metal ratios (up to 30:1), therefore a reinvestigation of the amino acid equilibria was performed to obtain precise values for the protonation and Cu-II complexation constants. The thermodynamic stereoselectivity in the formation of [CuLA] and [CuLH(-1)A](-) ternary complexes was significant in the case of both species with proline and for [CuLH(-1)A](-) of phenylalanine. This stereoselectivity was tentatively explained in terms of steric hindrance between tryptophanhydroxamate and amino acid residues. (C) 2001 Elsevier Science B.V. All rights reserved.