795216-66-9Relevant articles and documents
Electron transfer. 92. Reductions of vitamin B12a (hydroxocobalamin) with formate and related formyl species
Linn Jr.,Gould
, p. 1625 - 1628 (2008/10/08)
Vitamin B12a (hydroxocobalamin) is reduced to B12r (cob(II)alamin) with formate in aqueous media. One unit of formate consumes nearly two molecules of B12a. At formate concentrations below 0.1 M, reactions are first order in both reagents. Rates vary with pH, approach a maximum in the range pH 5-7, and conform to eq 4 in the text, indicating that the active species are the formate anion and the protonated form of B12a. At formate concentrations exceeding 0.1 M, the formate dependence exhibits kinetic saturation, pointing to the formation of a B12a-formate complex having Kassn = 4.6 M-1. The reaction is inhibited moderately by acetate and thiocyanate but severely by imidazole. The observed deuterium isotope effect, kHCOO-/kDCOO- = 1.8, is very close to that reported for the Cannizzaro reaction of benzaldehyde and is thus consistent with a path entailing migration of hydride from a formyl carbon to cobalt. The proposed mechanism for this reaction (sequence (6) - (9)) then features an internal hydride shift (k = 0.016 s-1) within a B12a-formate complex to yield a protonated CoI (B12s-like) intermediate, which very rapidly undergoes a comproportionation reaction with unreacted B12a. The reduction proceeds inconveniently slowly, or not at all, with a number of formyl-substituted carboxylic acids in which the aldehydo group is not properly positioned for hydride migration to carboxyl-bound CoIII or, in the case of glyoxylic acid, is nearly completely converted by hydration to its less reactive gem-diol form.
Electron transfer. 93. Further reactions of transition-metal-center oxidants with vitamin B12s (Cob(I)alamin)
Pillai, G. Chithambarathanu,Ghosh,Gould
, p. 1868 - 1871 (2008/10/08)
Vitamin B12, (cob(I)alamin) reduces europium(III), titanium(IV) (TiO(C2O4)22-), and uranium(VI) in aqueous solution. These oxidants undergo one-electron changes, leading in each case to the cobalt product cob(II)alamin (B12r). The reduction of Eu3+, which is inhibited by TES buffer, but not by glycine, is outer sphere. Its limiting specific rate (1 × 102 M-1 s-1), incorporated in the Marcus treatment, yields a B12s,B12r self-exchange rate of 104.8±0.5 M-1 s-1. Reductions of TiO(C2O4)22- are accelerated by H+ and by acetic acid. Kinetic patterns suggest three competing reaction paths involving varying degrees of protonation of the Ti(IV) center or its association with acetic acid. The very rapid reduction of U(VI) (k = 4 × 106 M-1 s-1) yields U(V) in several buffering media, even when B12s is taken in excess. The much slower conversion of U(V) to U(IV), although thermodynamically favored, appears to be retarded by the extensive reorganization of the coordination sphere of oxo-bound U(V) that must accompany its acceptance of an additional electron. The observed specific rate for the B12s-U(VI) reaction is in reasonable agreement, in the framework of the Marcus formalism, with reported values of the formal potential and the self-exchange rate for U(V,VI).
