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The acid‐catalyzed heterolytic cleavage of the Co‐C bond in coenzyme B12 (1) and adocobinamide hydroxide (2) was studied using the excess‐acidity functions of Cox and Yates. The results are compared to the acid‐induced decomposition of methyl(5′‐deoxyribofuranosyl)cobalamin (3) and (2‐ethoxyethyl)cobalamin (4). Evidence is presented to view the bond cleavage in the compounds 1, 2 and 3 as an A2 mechanism or as an Ia substitution at Co3+, with water as the nucleophile, and in 4 as an A1 mechanism. For 1, the temperature dependence (289‐320 K) gives ΔS‡ = − 96 J/(K· mol), in agreement with the Ia (A2) mechanism. For the A2 reactions the volumes of activation [ΔV‡ = −11.4 (1), −8.2 (2) and −11.5 (3) cm3/mol in 2‐4M acid at 298K] are consistent with the proposed mechanism, but ΔV‡ for the A1 mechanism of 4 is exceptionally negative (‐7.2 cm3 /mol), probably due to intramolecular coordination in the transition state. A comparison with the much slower acid‐induced decomposition of 5′‐deoxyadenosine brings out the labilization of the furanosyl oxygen bridge by coordination of the 5′‐deoxyadenosyl or 5′‐deoxyribofuranosyl group at the β‐position to cobalt. For 5′‐deoxyadenosine the excess‐acidity treatment and the activation volume (ΔV‡ = 1.7 cm3/mol in 2 M acid) indicate a clear A1 mechanism.

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Abstract.The acid-catalyzed heterolytic cleavage of the Co-C bond in coenzyme B,, (1) and adocobinamide hydroxide (2) was studied using the excess-acidity functions of Cox and Yates. The results are compared to the acid-induced decomposition of methyl(5'-deoxyribofuranosyl)cobalamin (3) and (2-ethoxyethy1)cobalamin (4). Evidence is presented to view the bond cleavage in the compounds 1, 2 and 3 as an A , mechanism or as an I , substitution at Co3+, with water as the nucleophile, and in 4 as an A , mechanism. For 1, the temperature dependence (289-320 K) gives A S s = -96 J/(K.mol), in agreement with the I , ( A , ) mechanism. For the A , reactions the volumes of activation [ A V s = -11.4 (I), -8.2 (2) and -11.5 (3) cm3/mol in 2-4M acid at 298K] are consistent with the proposed mechanism, but AVs for the A , mechanism of 4 is exceptionally negative ( -7.2 cm3/mol), probably due to intramolecular coordination in the transition state. A comparison with the much slower acid-induced decomposition of 5'-deoxyadenosine brings out the labilization of the furanosyl oxygen bridge by coordination of the 5'-deoxyadenosyl or 5'-deoxyribofuranosyl group at the P-position to cobalt. For 5'-deoxyadenosine the excess-acidity treatment and the activation volume (AV' = 1.7 cm3/mol in 2 M acid) indicate a clear A , mechanism.

Homolysis of B12 81 coenzyme in SummarySolvational changes on the transfer ofcoenzyme B~2 from water to 1-propanol/water mixtures were studied from the temperature dependence of the measured solubilities. These results were combined with calculations using the scaled particle theory. The strong decrease of the Gibbs energy of cavity formation on going from water to 1-propanol is for the smaller mole fractions of 1-propanol (x < 0.3) nearly balanced by a decreasing preferential (peripheral) solvation of the hydrophilic macromolecule by water. Thermodynamic and activation parameters were derived for the thermal homolytic Co--C bond breaking of coenzyme B12 in 50 wt% 1-propanol/water and compared with earlier values for the homolysis at 380.6 K in the mixtures. An initial state-transition state dissection of solvent effects for the homolysis reveals that desolvation does not play an important role in activation. The slightly exceptional position of water stems from a stabilization of the initial state.