Thermolysis of the Co-O Bond of Adenosylcobalamin. 2. Products, Kinetics, and Co-O Bond Dissociatiom Energy in Aqueous Solution

Article abstract of DOI:10.1021/ja00276a020

The reaction product, kinetic, ΔH^exct., ΗS^exct., 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 Co^IIB₁₂ 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 ΔH^exct._h=31.8 +/- 0.7 kcal/mol and ΔS^exct._h=6.8 +/- 1.0 eu.These values are significantly different from previously reported aqueous (pH 4.3) values of ΔH^exct._h=26.3 +/- 0.6 kcal/mol and ΔS^exct._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.