Kinetics of the formation and dissociation of pentacyanoferrate(II) complexes of cysteine, penicillamine, glutathione, and 2-mercaptoethylamine

Abstract

The results of a kinetic study of the formation and dissociation of pentacyanoferrate(II) complexes of cysteine (cys), gultathione (glut), penicillamine (pen), and 2-mercaptoethylamine (mea) are reported. Formation rate measurements have been made in the range pH 1–10 and the observed acid dependences are attributed to reaction of various ionic forms of the ligands related by protonation equilibria. For the neutral ligands the rate constants for formation (k_L) of complexes with cys, glut, pen, and mea are 330, 370, 350, and 200 dm³ mol^–1s^–1, respectively, at 25 °C and I= 0.50 mol dm^–3(Na[ClO₄]). A correlation between the formation rate constants and the ligand charge type is observed; deviations from this relationship are discussed in terms of the location of the charges in the ligand. Rates of ligand release have been measured over the range pH 2.5–11, in the presence of an excess of pyrazine, and a hydrogen-ion dependence observed for each ligand. A decrease in the dissociation rate with increasing pH is attributed to the reduced reactivity of [Fe(CN)₅(^–SR)]^n– with respect to its conjugate acid [Fe(CN)₅(HSR)]^(n–1)–. At pH 5 the rate constants for dissociation (k_–L) of cys, glut^–, pen, and Hmea⁺ from [Fe(CN)₅L]^n– are 0.104, 0.055, 1.34, and 0.102 s^–1, respectively. The large value of k_–L for penicillamine is associated with the steric effects of the methyl groups.