Photoinduced electron transfer (ET) between zinc myoglobin (ZnPPMb) and a variety of quenchers, such as hexacyanoferrate(III) ([Fe(CN)6]3−) and hexaammineruthenium(III) ([Ru(NH3)6]3+) ions, cationic viologens, copper(II) protein (stellacyanin), and metmyoglobins, has been studied in aqueous degassed solutions. The excited triplet state of ZnPPMb (*ZnPPMb) was quenched by [Fe(CN)6]3− in a self-associated complex. Both quenching rate constant and formation constant of the self-associated complex decrease with increasing ionic strengths. The thermal backward ET reaction for this system was not observed; it is most likely that the backward ET step is much faster than the quenching reaction. All of the cationic quenchers examined in this work did not form a self-associated complex with *ZnPPMb, and the intermolecular quenching occurred. The thermal backward ET reaction was observed for these cationic quenchers. Not only photoinduced ET but also thermal backward ET reactions were insensitive to the driving force of the reactions, suggesting that the reactions are controlled by conformational changes in ZnPPMb. The quenching rate constants increase with increasing ionic strength for the cationic quenchers. The effects of poly-l-lysine hydrochloride, sodium poly-l-glutamate, and sodium cyclo-hexaphosphate were also examined. The active site of the *ZnPPMb toward both anionic and cationic quenchers is assumed to be the positively charged site near the heme pocket.

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Both photoinduced electron-transfer and thermal backward electron-transfer reactions of zinc myoglobin with cationic quenchers were bimolecular steps and the reaction rates were insensitive to the driving force of reactions, suggesting that the reactions are controlled by conformational changes of zinc myoglobin.