Stabilization of the base-off forms of vitamin B12 and coenzyme B12 by encapsulation of the α-axial 5,6-dimethylbenzimidazole ligand with cucurbit[7]uril

Cucurbit[7]uril (CB[7]) forms very stable complexes with the alpha-axial 5,6-dimethylbenzimidazole (alpha-DMB) nucleotide base when dissociated from the Co(III) center in vitamin B-12 (CNCbl, K-CB[7] = (7.5 +/- 0.5) x 10(4) dm(3) mol(-1)) and coenzyme B-12 (AdoCbl, K-CB[7] = (3.02 +/- 0.35) x 10(6) dm(3) mol(-1)). The inclusion of alpha-DMB ligand facilitates its release from the metal center and its subsequent protonation, with significantly higher pK(base-off) values of 3.77 and 6.61, than determined for the free CNCbl (0.11) and AdoCbl (2.67), respectively Addition of CB[7] to the base-on form of CNCbl at pH 2 provides for a direct measurement of the rate constant for the dissociation of the alpha-DMB ligand from the Co center (k = (4.6 +/- 0.2) x 10(-2) s(-1), Delta H-double dagger = 93 +/- 2 kJ mol(-1), Delta S-double dagger = +41 +/- 6 J K-1 mol(-1)). The beta-axial 5'-deoxyadenosyl ligand (beta-Ado) on coenzyme B-12 is bound more weakly by a second CB[7] host (K-CB[7] = (1.1 +/- 0.2) x 10(3) dm(3) mol(-1)), however inclusion of the beta-Ado ligand has no effect on the kinetics of its heterolytic photodissociation from the Co(III) center.