Reported herein are the synthesis and improved purification of MeCbi(+).BF4- leading to 95% pure product. The availability of this higher purity MeCbi(+).BF4- has, in turn, allowed a study of the K-assoc, DeltaH, and DeltaS for exogenous imidazole and pyridine bases binding to MeCbi(+) in ethylene glycol and buffered aqueous solution. The results show that (1) the bases studied have larger K-assoc values (where measurable) when binding to MeCbi(+) than when binding to AdoCbi(+) under analogous conditions; (2) comparison of the thermodynamic binding parameters for py and N-Melm show that these bases bind similarly, within experimental error to MeCbi(+), contrary to what was seen earlier with AdoCbi(+); (3) the bases follow the expected trend, with the base with the highest pK(a) of those studied, 4-Me(2)Npy, exhibiting the highest K-assoc value (K-assoc(25 degreesC) = 18.0 +/- 0.3 M-1) and the base of lowest pK(a), py, exhibiting the lowest detectable K-assoc value (K-assoc (25 degreesC) 6.2 +/- 0.4 M-1); (4) there is no detectable binding (K-assoc = 0.07 M-1) for 2-Mepy or 2,6-Me(2)py with MeCbi(+); and (5) the base that is closest to the biologically relevant axial His759 residue in methionine synthase, N-Melm, exhibits an unusual DeltaH value for the formation of MeCbi(+).N-Melm, results interpreted as offering further support for the presence of sigma plus pi effects when imidazole bases bind to alkylcobinamides. The results of these studies allow the percentage of base-on methylcobinamide, MeCbi(+)-base, to be calculated as a function of temperature and added base. As such, they provide necessary background information for RS- + MeCbi(+).base and other methionine synthase chemical precedent studies.