ic951272j_si_001.pdf (546.19 kB)
Download fileAdenosylcobinamide, 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
journal contribution
posted on 1996-09-25, 00:00 authored by Cheryl D. Garr, Richard G. FinkeAdenosylcobinamide
(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.