title: Stabilization of the base-off forms of vitamin B(12) and coenzyme B(12) by encapsulation of the alpha-axial 5,6-dimethylbenzimidazole ligand with cucurbit[7]uril.
abstract: 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), DeltaH = 93 +/- 2 kJ mol(-1), DeltaS = +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.
authors.affiliation: Department of Chemistry, Queen's University, Kingston, ON K7L 3N6, Canada.
authors.forename: Ruibing
authors.initials: R
authors.lastname: Wang
authors.forename: Brendan C
authors.initials: BC
authors.lastname: Macgillivray
authors.forename: Donal H
authors.initials: DH
authors.lastname: Macartney
mesh_terms: D001562:Benzimidazoles / Q000737:chemistry*; D001952:Bridged-Ring Compounds / Q000737:chemistry*; D003038:Cobamides / Q000737:chemistry*; D006863:Hydrogen-Ion Concentration; D007093:Imidazoles / Q000737:chemistry*; D007700:Kinetics; D009682:Magnetic Resonance Spectroscopy; D010782:Photolysis; D013056:Spectrophotometry, Ultraviolet; D017186:Titrimetry; D014805:Vitamin B 12 / Q000737:chemistry*; D047028:Macrocyclic Compounds; D000099246:Cucurbit(n)urils
chemical_list: D001562:Benzimidazoles; D001952:Bridged-Ring Compounds; D003038:Cobamides; D007093:Imidazoles; D014805:Vitamin B 12; C456276:cucurbit(7)uril; D047028:Macrocyclic Compounds; D000099246:Cucurbit(n)urils; C015158:5,6-dimethylbenzimidazole; C000913:cobamamide