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 K_association, ΔH, and ΔS Measurements plus a Critical Review of the Relevant Biochemical Literature

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, K_assoc, 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 K_assoc studies are 2-fold:  the more electron donating the base, the greater the K_assoc, and bulky bases bind weakly if at all. This paper also contains a tabular summary of the existing, non-Ado RCbi⁺ axial-base K_assoc literature plus the relatively few associated ΔH and ΔS values that are available. Selected B₁₂ model axial-base K_assoc 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.