Περίληψη:
Vanadium(II) and vanadium(III) form with cysteine (cys) and other sulphhydryl compounds intensely yellow complexes soluble in neutral and weakly alkaline solutions. Some of the vanadium(II) complexes are powerful reductants. Thus, VII-cys reduces water to dihydrogen under mild conditions. The formation of the reducing species, which is [VII(cysOS)3]4- [cysOS = cysteinate(2-)] proceeds in two stages, i.e. a stage corresponding to a jump in the absorbance at zero time and a second stage, which was followed kinetically in a stopped-flow instrument. The complexation of cysteine with vanadium(III) also proceeds in two stages, but leads to the formation of a species containing two cysteines instead of three (at pH values around neutral). The observed activation energy (Ea) and pre-exponential factor (A) for the stages that were followed kinetically are as follows: Ea = 41 ± 4 kJ mol-1, A = 1.4 × 107 (pH 8.2) for VII and Ea = 41 ± 4 kJ mol-1, A = 1.4 × 1010 (pH 8.8)for VIII. The reduction of water to dihydrogen by VII-cys proceeds with a rate first order in [VII]total and in the pH range 7.5-8.5 it is independent of hydrogen-ion concentration. The activation parameters are: Ea = 54 ± 2 kJ mol-1, A = 5 × 106. Dihydrogen is also obtained with VII-cys (cysa = cysteamine) and VII-cysme (cysme = cysteine methyl ester). The corresponding reaction of VII-ser (ser = serine) is ca. one thousand times slower compared to the reaction of VII-cys, even though the polarographic half-wave potentials have comparable values.
Συγγραφείς:
Konstantatos, J.
Kalatzis, G.
Vrachnou-Astra, E.
Katakis, D.
