Nuclear magnetic resonance relaxation studies on partially reduced solutions of Candida krusei and horse heart cytochrome c reveal diversity in their functional behavior. Electron transfer between the two oxidation states in vitro proceeds at a rate nearly an order of magnitude slower in Canadida krusei than in the horse heart protein. This difference is interpreted in terms of an active role of the polypeptide chain in the electron transfer process since the hemes in the two proteins are identical in chemical structure and similar electronically. In both cases, electrostatic repulsion between positively charged protein molecules contributes significantly to the observed free energy of activation for the electron transfer process. © 1973.