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 voltammet ry, H-1 NMR and circular dichroism measurements in mixed water-dimethy lsulfoxide (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 affect s to some extent the conformation of alkaline cyt c and, notably, stab ilizes both redox states of this form to the detriment of the native f orm. In particular, DMSO lowers the pK(a) of the native to alkaline tr ansition for oxidized cyt c and increases the electrochemical reversib ility of the voltammetric wave of the alkaline form. DMSO-induced chan ges in the reduction entropy for native and alkaline cyt c were also d etermined and interpreted tentatively in terms of solvation properties of the heme and structural features of the protein environment. (C) 1 998 Elsevier Science S.A. All rights reserved.