3415-89-2Relevant articles and documents
Thermolysis of the Co-O Bond of Adenosylcobalamin. 2. Products, Kinetics, and Co-O Bond Dissociatiom Energy in Aqueous Solution
Hay, Benjamin P.,Finke, Richard G.
, p. 4820 - 4829 (1986)
The reaction product, kinetic, ΔHexct., ΗSexct., and Co-O bond dissociation energies (BDEs) are reported for the anaerobic, thermal decomposition of adenosylcobalamin (AdoB12) in aqueous solution.These studies reveal that the reaction proceeds via two competing pathways, heterolytic Co-O bond clevage to yield aquocobalamin, adenine, and a sugar residue and competing Co-O bond homolysis to give CoIIB12 and 8,5'-anhydroadenosine.At pH 4.0, heterolysis is the major mode (88percent at 85 deg C) of decomposition, while at pH 7.0 homolysis dominates (90percent at 85 degC).The temperature dependence of the rate of AdoB12 Co-O bond homolysis in neutral H2O was obtained from 85.0 to 110.0 deg C, yielding ΔHexct.h=31.8 +/- 0.7 kcal/mol and ΔSexct.h=6.8 +/- 1.0 eu.These values are significantly different from previously reported aqueous (pH 4.3) values of ΔHexct.h=26.3 +/- 0.6 kcal/mol and ΔSexct.h=-6 +/- 2 eu.The temperature dependence of the axial base equilibrium of AdoB12 in neutral water was measured, yielding ΔH=-5.6 +/- 0.9 kcal/mol and ΔS=-13 +/- 3 eu.Combining the above results with other data yields an estimate of 30 +/- 2 kcal/mol for the base-on Co-O BDE of AdoB12 in water, in good agreement with our previously reported value of 31.5 +/- 1.3 kcal/mol obtained in ethylene glycol.
Synthesis and characterization of 3',4'-anhydroadenosylcobalamin: A coenzyme B12 analogue with unusual properties
Magnusson,Frey
, p. 8807 - 8813 (2007/10/03)
The question of how coenzyme B12-Dependent enzymes facilitate the cleavage of the Co-C bond of the cofactor is of interest. We have synthesized an analogue of 5'-Deoxyadenosylcobalamin (AdoCbl1) designed to stabilize the 5'-Deoxyadenosyl radical (5'-Deoxyadenosine-5'-Yl) that is produced upon homolysis of the Co-C bond. By replacement of the upper axial ligand of AdoCbl by a 3',4'-Anhydro-5'-Deoxyadenosyl moiety, the radical formed on the nucleoside analogue is stabilized by allylic delocalization. The compound, 5'-Deoxy- 3',4'-Anhydroadenosylcobalamin (3',4'-AnAdoCbl) was synthesized by chemical and enzymatic methods. The final step was coupling of cob(I)alamin and 3',4'-AnhydroATP catalyzed by CobA, an ATP: corrinoid adenosyltransferase. 3',4'-AnAdoCbl displays interesting properties. The compound has not been purified to homogeneity due to its thermal and oxygen sensitivity. It was characterized by UV-Vis spectroscopy, ESI-MS, and NMR spectroscopy. The bond dissociation energy of the Co-C bond of the analogue was measured by radical trapping techniques. A significantly weaker bond (24 ± 2 kcal/mol) as compared to AdoCbl (30 kcal/mol) was observed, as was homolytic cleavage at ambient temperature. Photolysis experiments conducted under anaereobic conditions reveal no formation of cob(II)alamin, whereas the compound breaks down rapidly under aerobic conditions as measured by cob(III)alamin formation. We postulate that the weak Co-C bond is cleaved reversibly by photolysis, where recombination of the allylic radical and cob(II)alamin occurs efficiently in the absence of a radical scavanger. Activation of the coenzyme B12-Dependent enzymes dell dehydrase and ethanolamine ammonia-Lyase was observed with the cofactor analogue. The measured activity was low and no formation of cob(II)alamin could be detected in the steady-State of the reaction for either enzyme. Comparative interactions of AdoCbl and 3',4'-An AdoCbl with dell dehydrase and ethanolamine ammonia-Lyase suggest that cleavage of the Co-C bond is facilitated by enzyme-coenzyme binding contacts that are remote from the Co-C bond.