The kinetics of reduction of cytochrome c by catechol(s), quinol(s) and related compounds were investigated by stopped-flow spectrophotometry. Studies on the influence of pH on the rates indicate that only deprotonated forms of these compounds act as reducing agents, with the dianionic forms being the most effective. The pH-independent second-order rate constants are reported. Hammett treatment of the effects of substituents on the aromatic ring structure of the reductants show that for electron transfer to occur the charge on the deprotonated species must not be withdrawn on to the substituents. Possible sites for electron donation to cytochrome c are discussed, and the results indicate that the haem edge is a likely candidate.

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