Triplet-state kinetics of Zn-porphyrin cytochrome c in micellar media: measurement of intermicellar exchange rates

The interactions of protein molecules with surfactant assemblies in aqueous and hydrocarbon media have been studied via the triplet-state kinetics of Zn-porphyrin cytochrome c in solutions containing an anionic [sodium bis(2-ethylhexyl)sulfosuccinate, AOT] or a cationic (cetyltrimethylammonium bromide, CTAB) surfactant. In aqueous solution, the observed triplet state decay is single exponential with a lifetime of 8 ms. In aqueous solutions of AOT and in AOT-reversed micellar solutions, biexponential triplet state decays were observed, indicating that interactions between the surfactant and the protein occur, resulting in a change in protein conformation near the porphyrin ring. In CTAB-reversed micellar solutions, quenching of the Zn-porphyrin cytochrome c triplet state by ferricyanide and methyl viologen was studied. Because the quenching is exchange-limited under the conditions used, the exchange rate constants for the water pools can be obtained from these experiments. The observed exchange rate constants are in the range of (1–5) × 107 M−1 s−1, depending on the water content of the reversed micelles and on the type of quencher used. These values are three orders of magnitude lower than the calculated collision rate of the reversed micelles.