Mechanisms and kinetics of the oxidation of thiols in anaerobic aqueous media.

摘要

The oxidation of thioglycolic acid (TGA) by [Os(phen) 3]3+ (phen = 1,10-phenanthroline) was catalyzed by traces of ubiquitous Cu2+ and inhibited by the product Os(II). In the presence of dipicolinic acid (dipic), which thoroughly hinders trace Cu 2+ catalysis, and spin trap PBN, the kinetics under anaerobic conditions have been studied in the pH range 1.82--7.32. The rate law is - d[Os(III)]/dt = k[TGA]tot[Os(III)], k=2&cubl0;&parl0;kbK a1+kcKa1K i&parr0;&sqbl0;H+&sqbr0;+k dKa1Ka2 &cubr0;&sqbl0;H+&sqbr0;2+&parl0; Ka1+Ka1K i&parr0;&sqbl0;H+&sqbr0;+K a1Ka2; Ka1 and K a2 are the successive acid dissociation constants of TGA, and Ki is the tautomerization constant of two TGA mono-anions. kb + kcK i = (1.12 +/- 0.06) x 104 M-1 s-1, kd = (1.33 +/- 0.04) x 109 M-1 s -1 at mu = 0.1 M (NaCF3SO3) and 25°C. UV-vis spectra showed that [Os(phen)3]2+ was not the only Os-containing product. The stoichiometric ratio of 1.46 was determined at pH 7.5 in the absence of spin trap and 2 at pH 4.9 in the presence of DMPO. A mixture of S-containing products observed in the 1H NMR spectrum suggests the occurrence of a rather complicated oxidation reaction relative to formation of disulfide, which was then confirmed by electro-spray mass spectrometry (ESMS) detection of OsIIIL2L-RS (L = phenanthroline, L-RS = substitution of RS- on phenanthroline).; Powerful catalysis by traces of ubiquitous Cu2+ accelerated the oxidation of L-cysteine by [Mo(CN)8] 3- by about 40 times at pH 4.2. With 2 mM PBN present, this reaction rate was suppressed about 12% with pseudo-first-order behavior. A strong sodium ion effect was also observed on this reaction. In the presence of dipicolinic acid (dipic), which completely impedes trace Cu2+ catalysis, and spin trap PBN as radical scavenger, the kinetics were studied in anaerobic conditions over the pH range 3.48--12.28 with [Na +] maintained at 0.09--0.10 M. The rate law is -d[Mo(V)]/ dt = k[L-cysteine]tot[Mo(V)], k=2&parl0;kbKa1 Ka2+kcK a1Ka2Ki&parr0; &sqbl0;H+&sqbr0;+kdK a1Ka2Ka3 &sqbl0;H+&sqbr0;3+K a1&sqbl0;H+&sqbr0;2+ Ka1Ka2&sqbl0; H+&sqbr0;+Ka1Ka2 Ka3 Ka1, Ka2 and Ka3 are the successive acid dissociation constants of HSCH2CH(NH3+)CO2H, and Ki is the tautomerization constant of two L-cysteine mono-anions.; kb + kc Ki = (6.9 +/- 0.4) x 104 M -1 s-1, kd = (2.3 +/- 0.2) x 104 M-1 s -1 at mu = 0.1 M (NaCF3SO3) and 25°C. UV-vis spectra and electrochemistry show [Mo(CN)8]4- is the Mo-containing product. The stoichiometric ratio (Delta nOs(III)/DeltanTGA) of 1.4 in absence of a spin trap at pH 10.8 and a mixture of S-containing identified products observed in 1H NMR spectrum suggest the occurrence of over-oxidation of L-cysteine.