ADENOSYLCOBINAMIDE, THE BASE-FREE ANALOG OF COENZYME B-12 (ADENOSYLCOBALAMIN) .1. PROBING THE ROLE OF THE AXIAL 5,6-DIMETHYLBENZIMIDAZOLE BASE IN COENZYME B-12 VIA EXOGENOUS AXIAL BASE K-ASSOCIATION, DELTA-H AND DELTA-S MEASUREMENTS PLUS A CRITICAL-REVIEW OF THE RELEVANT BIOCHEMICAL LITERATURE

Adenosylcobinamide (AdoCbi(+)BF(4)(-)), the base-free form of adenosyl cobalamin (AdoCbl or coenzyme B-12), has been studied with a series of 14 exogenous alpha-axial bases, Specifically, equilibrium association constants, K-assoc, as a function of temperature were measured, and t hus their associated Delta H and Delta S were obtained. Bases studied include the following: (i) exogenous 1,5,6-trimethylbenzimidazole [ana logous to adenosylcobalamin's intramolecularly appended 5,6-dimethylbe nzimidazole base]; (ii) sterically encumbered phosphine bases (none of which showed detectable binding in dramatic contrast to studies of, f or example, cobaloxime B-12 ''models''); and (iii) electronically incr easingly donating, but isosteric, 4-substituted pyridine axial bases. The general trends from the present K-assoc studies are 2-fold: the mo re electron donating the base, the greater the K-assoc, and bulky base s 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 Delta H and Delta S values that are availabl e. Selected B-12 model axial-base K-assoc literature is also summarize d as Supporting Information. In addition, the Discussion contains a cr itical analysis of the prior, B-12 enzymic biochemical literature rele vant to the role of AdoCbl's appended 5,6-dimethylbenzimidazole axial base.