Reduction of ferricytochrome c catalyzed by optically active chromium(III) complexes

The reduction rates of horse heart ferricytochrome c by amalgamated zinc or by electrolysis at fixed potential on a mercury pool as the cathode have been measured in a buffered solution at pH 7.5 by absorption spectrophotometry. In both cases, the reaction was strongly accelerated by the presence of the optically active complexes Lambda-[Cr(III)((S,S)-promp)H(2)O](+) (H(2)promp = N,N'-[(pyridine-2,6-diyl)bis(methylene)]-bis[(S)-proline]), Delta-[Cr(III)((R,R)-alamp)H(2)O](+) (H(2)alamp = N,N'-[(pyridine-2,6-diyl)bis(methylene)]-bis[(R)-alanine]) and Lambda-[Cr(III)((S,S)-alamp)(H(2)O)(2)](+). These were shown to undergo reversible one-electron reduction to the corresponding labile chromium(II) species by cyclic voltammetry (CV), although the diaquo Lambda-[Cr(III)((S,S)-alamp)(H(2)O)(2)](+) compound behaved differently than the two others. The cyclic voltammogram evidenced a strong catalytic reduction wave below -1.1 V/SHE overlapping with the Cr(3+)/Cr(2+) couple, which has been attributed to the catalytic reduction of hydroxonium ions to molecular hydrogen. Although stable in the second time range as demonstrated by CV, the chromium(II) complexes exist in solution only as short-lived species in the absence of protein and are rapidly reoxidized to the initial trivalent state, thus preventing their isolation even under anaerobic conditions. However, their lifetime was found to be long enough to catalyze the reduction of the ferric heme moiety of cytochrome c according to an electron-transfer-mediated reaction. Both chemical and electrochemical processes were found to follow zero-order kinetics. It could therefore be safely concluded that the rate-determining step is associated to the electron transfer from transient chromium(II) complexes to the protein and not to the in situ generation of the metallic reducing agent.