Effects of solvent on the redox properties of cytochrome c: cyclic voltammetry and H-1 NMR experiments in mixed water-dimethylsulfoxide solutions

Bovine heart cytochrome c (cyt c) was studied through cyclic voltammetry, H-1 NMR and circular dichroism measurements in mixed water-dimethylsulfoxide (DMSO) solutions containing up to 50% DMSO by volume, under different conditions of temperature and pH. The effect of DMSO on the reduction potential of native cyt c was found to be determined mainly by the decrease in dielectric constant of the medium. No appreciable specific DMSO-protein interactions were detected. Instead, DMSO affects to some extent the conformation of alkaline cyt c and, notably, stabilizes both redox states of this form to the detriment of the native form. In particular, DMSO lowers the pK(a) of the native to alkaline transition for oxidized cyt c and increases the electrochemical reversibility of the voltammetric wave of the alkaline form. DMSO-induced changes in the reduction entropy for native and alkaline cyt c were also determined and interpreted tentatively in terms of solvation properties of the heme and structural features of the protein environment. (C) 1998 Elsevier Science S.A. All rights reserved.