Equilibrium and Kinetic Studies of the Iron(III) Complex with N-Pyridoxylidene-l-tryptophan: Electronic Absorption and Circular Dichroism Spectral Characteristics, Stability, and Formation and Ligand-Substitution Reaction Mechanisms

Abstract

Abstract Studies have been made of the electronic absorption and circular dichroism (CD) spectral characteristics, thermodynamic stability, and formation and ligand-substitution reaction kinetics of an iron(III) complex with N-pyridoxylidene-l-tryptophan (Hpdx-l-Htrp). The 1:2 (metal: ligand) complex forms in an acidic aqueous solution with the stability constant (β2) of logβ2=18.38±0.10 mol−2 dm6 (0.50 mol dm−3 (NaCl), 25 °C). The complex shows a characteristic CD spectrum in the charge-transfer region. The kinetics were studied by a stopped-flow CD method in a 0.50 mol dm−3 aqueous NaCl solution at 25 °C. The formation of the complex proceeds through six parallel pathways of Fe3+, FeOH2+, and FeCl2+ with H2pdx-l-Htrp+ and H2pdx-l-trp, with the coordination of the carboxylato group to iron(III) as the rate-determining step. The ligand-substitution reaction of the complex by ethylenediamine-N,N,N′,N′-tetraacetic acid depends neither on the entering ligand nor on the hydrogen-ion concentration. The dissociation of a coordinated Hpdx-l-trp− from [Fe(Hpdx-l-trp)2]+ is concluded to be the rate-determining step. The reaction mechasnisms are discussed in detail.