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

Abstract

Cucurbit[7]uril (CB[7]) forms very stable complexes with the α-axial 5,6-dimethylbenzimidazole (α-DMB) nucleotide base when dissociated from the Co(III) center in vitamin B₁₂ (CNCbl, K_CB[7] = (7.5 ± 0.5) × 10⁴ dm³ mol⁻¹) and coenzyme B₁₂ (AdoCbl, K_CB[7] = (3.02 ± 0.35) × 10⁶ dm³ mol⁻¹). The inclusion of α-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 α-DMB ligand from the Co center (k = (4.6 ± 0.2) × 10⁻² s⁻¹, ΔH^‡ = 93 ± 2 kJ mol⁻¹, ΔS^‡ = +41 ± 6 J K⁻¹ mol⁻¹). The β-axial 5′-deoxyadenosyl ligand (β-Ado) on coenzyme B₁₂ is bound more weakly by a second CB[7] host (K_CB[7] = (1.1 ± 0.2) × 10³ dm³ mol⁻¹), however inclusion of the β-Ado ligand has no effect on the kinetics of its heterolytic photodissociation from the Co(III) center.