The kinetics and mechanism of cobalamin-dependent methyl and ethyl transfer to mercuric ion |
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Authors: | R.E. DeSimone M.W. Penley L. Charbonneau S.G. Smith J.M. Wood H.A.O. Hill J.M. Pratt S. Ridsdale R.J.P. Williams |
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Affiliation: | 1. Biochemistry and Chemistry Departments, School of Chemical Sciences, University of Illinois, Urbana, Ill., 61801 U.S.A.;2. Inorganic Chemistry Laboratory, Oxford Great Britain |
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Abstract: | The chemical transfer of alkyl groups from alkylcobalamins to mercuric ion has been studied in detail by using ultraviolet-visible conventional and stopped-flow kinetics and, in the case of methyl group transfer, by 220 MHz NMR spectroscopy. These experiments show that heterolytic cleavage of the Co–C δ-bond occurs during electrophilic attack by mercuric ion to give alkylmercury and aquocobalamin as the reaction products. Equilibrium and kinetic experiments are consistent with the initial displacement of 5,6-dimethylbenzimidazole by mercuric ion which results in a deactivaion toward dealkylation by a second mercuric ion. Consequently the main dealkylation reaction at pH 5.0 occurs with uncomplexed alkylcobalamin with the overall rate kobd being controlled by the above equilibrium. Both the displacement of 5,6-dimethylbenzimidazole (“fast reaction”) and dealkylation (“slow reaction”) are first order in the active mercuric species. |
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Keywords: | To whom to address the correspondence. |
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