Journal of the American Chemical Society p. 6553 - 6558 (1989)
Update date:2022-08-12
Topics:
Pleasants, Joan C.
Guo, Wei
Rabenstein, Dallas L.
The kinetics of symmetrical selenol/diselenide and thiol/disulfide exchange reactions involving selenocysteamine/selenocystamine and cysteamine/cystamine have been studied in D2O solution by NMR spectroscopy.The rate of selenol/diselenide exchange is so fast that resonances for the selenol and diselenide forms are coalesced in (1)H NMR spectra of millimolar selenocysteamine/selenocystamine mixtures at pD > 2-3.In contrast, the rate of thiol/disulfide exchange is so slow that separate, sharp resonances are observed for both cysteamine and cystamine in mixtures atconcentrations up to at least 0.2 M from pD < 1 to > 13.Rate constants for the selenol/diselenide exchange reaction were determined by line shape analysis of exchange-broadened resonances, while those for thiol/disulfide exchange were determined by an inversion-transfer method.The rate constants at 25 deg C for exchange by reaction of D3N(1+)CH2CH2X(1-) with D3N(1+)CH2CH2XXCH2CH2ND3(1+) are as follows: X = Se, k = 1.65 * 1E7 L/mol*s; X = S, k = 68.0 L/mol*s.When the differences in the acidities of the selenol and thiol groups are accounted for, selenocysteamine/selenocystamine exchange is 1.2 * 1E7 times faster than cysteamine/cystamine exchange at physiological pH.
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