Dielectric constant dependence of biological oxidation-reduction. 1. A model of polarity-dependent ferrocytochrome c oxidation.
A theoretical model for the effect of the dielectric constant (c) of the solvent medium on ferrocytochrome c oxidation by ferricyanide is developed to account for the observed variations of the rate constant (k) of reactions in aqueous binary mixtures with alcohols (less than 5-10 mol% ethanol and propranolol). A correlation between k and c is found if ln k is expressed as a function of the Kirkwood parameter (c-1)(2c+1). The results of calculations indicate that the use of the 'overall dipole moment' of cytochrome c in oxidoreduction studies is likely to be unreliable. Instead, the decrease in k in alcohol/water mixtures is best explained--in conformity with Onsager's theory of the reaction field--by a polarity effect on the dipole moment of the cytochrome c heme upon diffusion of the polar solvent molecules into the low dielectric constant heme crevice.
D000438:Alcohols; D000818:Animals; D003574:Cytochrome c Group / Q000378:metabolism*; D006736:Horses; D007700:Kinetics; D008433:Mathematics; D008954:Models, Biological; D010084:Oxidation-Reduction
D000438:Alcohols; D003574:Cytochrome c Group