Solvation and cobalt-carbon homolysis of coenzyme B12 in 1-propanol/water mixtures

Abstract

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.