American Chemical Society
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Adenosylcobinamide, the Base-Free Analog of Coenzyme B12 (Adenosylcobalamin). 1. Probing the Role of the Axial 5,6-Dimethylbenzimidazole Base in Coenzyme B12 via Exogenous Axial Base Kassociation, ΔH, and ΔS Measurements plus a Critical Review of the Relevant Biochemical Literature

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journal contribution
posted on 1996-09-25, 00:00 authored by Cheryl D. Garr, Richard G. Finke
Adenosylcobinamide (AdoCbi+BF4-), the base-free form of adenosylcobalamin (AdoCbl or coenzyme B12), has been studied with a series of 14 exogenous α-axial bases. Specifically, equilibrium association constants, Kassoc, as a function of temperature were measured, and thus their associated ΔH and ΔS were obtained. Bases studied include the following:  (i) exogenous 1,5,6-trimethylbenzimidazole [analogous to adenosylcobalamin's intramolecularly appended 5,6-dimethylbenzimidazole base]; (ii) sterically encumbered phosphine bases (none of which showed detectable binding in dramatic contrast to studies of, for example, cobaloxime B12 “models”); and (iii) electronically increasingly donating, but isosteric, 4-substituted pyridine axial bases. The general trends from the present Kassoc studies are 2-fold:  the more electron donating the base, the greater the Kassoc, and bulky bases bind weakly if at all. This paper also contains a tabular summary of the existing, non-Ado RCbi+ axial-base Kassoc literature plus the relatively few associated ΔH and ΔS values that are available. Selected B12 model axial-base Kassoc literature is also summarized as Supporting Information. In addition, the Discussion contains a critical analysis of the prior, B12 enzymic biochemical literature relevant to the role of AdoCbl's appended 5,6-dimethylbenzimidazole axial base.