Adenosylcobinamide, the Base-Free Analog of Coenzyme B₁₂(Adenosylcobalamin). 1.¹Probing the Role of the Axial 5,6-Dimethylbenzimidazole Base in Coenzyme B₁₂via Exogenous Axial BaseKassociation, ΔH, and ΔSMeasurements plus a Critical Review of the Relevant Biochemical Literature

Adenosylcobinamide (AdoCbi + BF 4 - ), the base-free form of adenosylcobalamin (AdoCbl or coenzyme B 12 ), 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 Δ H and Δ S 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 B 12 “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 Δ H and Δ S values that are available. Selected B 12 model axial-base K assoc literature is also summarized as Supporting Information. In addition, the Discussion contains a critical analysis of the prior, B 12 enzymic biochemical literature relevant to the role of AdoCbl's appended 5,6-dimethylbenzimidazole axial base.