Περίληψη:
Mercaptoacetate in large excess and in the pH range between 3 and 10 forms with VIII intensely yellow complexes, in two successive stages, the second of which was followed by stopped-flow methods at pH 3.6 and found to be first order in vanadium(III) and in mercaptoacetate. The activation parameters are ΔH = 57.4 kJ mol-1 and ΔS‡ ‘0.8 J mol-1 K-1. Under similar conditions VII undergoes also a two-stage complexation reaction but reacts further, giving succinic acid and hydrogen sulfide. This redox reaction is first order in vanadium(II) and in mercaptoacetate and at pH 9.6 has ΔH‡ = 50.2 kJ mol-1 and ΔS‡ = -70 J mol-1 K-1. The second-order rate constant correlates with the titration curve of mercaptoacetic acid. The results are compared to analogous results for cysteine and are interpreted on the basis of a two-step chelate ring formation involving the S″ group. The activation of the VII and VIII reactions is partly attributed to proton dissociation of the organic moiety and partly to a strain in the ring, which when combined with electron transfer leads to a break of the C-S bond. The VII-mercaptoacetate system can be regarded as an electron-storing device, which can be activated by changing the proton environment. © 1983, American Chemical Society. All rights reserved.
Συγγραφείς:
Konstantatos, J.
Kalatzis, G.
Vrachnou-Astra, E.
Katakis, D.
