Theoretical studies (J. Inorg. Biochem. 2001, 83, 121) of the involvement of the bulky 5,6-dimethylbenzimidazole (Dmbz) ligand of coenzyme B-12, (5 ' -deoxyadenosylcobalamin, AdoCbl) in the mechanism of activation of the carbon-cobalt bond of the coenzyme for homolytic cleavage by AdoCbl-dependent enzymes (the "mechano-chemical triggering" mechanisms) have shown that a purely steric, ground-state mechanism can supply only a few kilocalories per mole (of the observed 13-16 kcal mol(-1)) of activation, but that an electronic mechanism, operating to, stabilize the transition state, can explain all of the observed catalytic effect. To address these mechanisms experimentally, analogues of AdoCbl in which the Dmbz ligand is replaced by benzimidazole (Ado(Bzim)Cbl) or by imidazole (Ado(Im)Cbl) have been prepared and characterized. Both of these analogues support turnover in the AdoCbl-dependent ribonucleoside triphosphate reductase (RTPR) from Lactobacillus leichmannii at 100%, of the activity of AdoCbl itself, but, the Ado(Im)Cbl analogue has a significantly higher K-m. 5'-Deoxyadenosylcobinamide, the analogue in which the axial nucleotide has been chemically removed, in contrast, is inactive in the spectrophotometric assay, which indicates that it has at most 1% of the activity of AdoCbl, Stopped-flow spectrophotometric measurements, of the formation of cob(II)alamin at the enzyme active site show that RTPR binds Ado(Bzim)Cbl slightly more weakly than it does AdoCbl, but binds Ado(Im)Cbl 8-fold, more weakly.While the equilibrium constant for cob(II)alamin formation is nearly the same for Ado(Bzim)Cbt and AdoCbl, it is 5-fold smaller for Ado(Im)Cbl. Finally, the forward rate constant for enzyme-induced Co-C bond homolysis was about the same for Ado(Bzjm)Cbl and for AdoCbl but was 17-fold smaller for Ado(im)Cbl. These results are consistent with a small contribution from ground-state mechanochemical triggering, but they do not in themselves rule out transition-state mechanical triggering.