| Abstract
| - Axial-base equilibrium association constants,Kassoc, and their associated ΔH andΔS parameters have been measured foradenosylcobinamide(AdoCbi+BF4-), thebase-free analog of adenosylcobalamin (AdoCbl or,equivalently, coenzyme B12), plus a series of 14 exogenousbases. The results are compared to tables of literatureKassoc and ΔH and ΔSvalues for other alkylCbi+. Also presented, as partof the Discussion, is a critical analysis of prior, controversial,B12 enzymic biochemical literature related to the role ofcoenzyme B12's appended5,6-dimethylbenzimidazole axial base. Theresults provide a more uniform picture for the previously controversialrole of the nucleotide loop and its appended axial5,6-dimethylbenzimidazole base inAdoB12.
- Adenosylcobinamide(AdoCbi+BF4-),the base-free form of adenosylcobalamin (AdoCbl or coenzymeB12), hasbeen 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 associatedΔH and ΔS were obtained. Basesstudiedinclude the following: (i) exogenous 1,5,6-trimethylbenzimidazole[analogous to adenosylcobalamin's intramolecularly appended 5,6-dimethylbenzimidazole base]; (ii) stericallyencumbered phosphine bases (none of whichshowed detectable binding in dramatic contrast to studies of, forexample, cobaloxime B12 “models”); and(iii)electronically increasingly donating, but isosteric, 4-substitutedpyridine axial bases. The general trends fromthe present Kassoc studies are 2-fold: themore electron donating the base, the greater theKassoc, and bulky basesbind weakly if at all. This paper also contains a tabular summaryof the existing, non-Ado RCbi+axial-baseKassoc literature plus the relatively fewassociated ΔH and ΔS values that areavailable. Selected B12 modelaxial-base Kassoc literature is also summarizedas Supporting Information. In addition, the Discussion containsacritical analysis of the prior, B12 enzymic biochemicalliterature relevant to the role of AdoCbl's appended 5,6-dimethylbenzimidazole axial base.
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