PHOTOINDUCED ELECTRON-TRANSFER IN SELF-ASSOCIATED COMPLEXES OF SEVERAL UROPORPHYRINS AND CYTOCHROME-C

Photoinduced electron transfer between cytochrome c and free base and metallouroporphyrin (Up, MUp) has been studied. Difference absorption spectrophotometry showed that the electrostatic interactions between Up and cytc(III) result in their forming a self-associated 1:1 complex in the ground state with a binding constant that depends upon the ionic strength. In the complex, the photoexcited uroporphyrin singlet state was quenched through a static interaction with the protein. Even under the most favorable quenching conditions, i.e., when all porphyrin was complexed, residual fluorescence was noted. More significantly the excited singlet state of the complex was shown to undergo small, but significant, intersystem crossing. These triplet states rapidly underwent an electron-transfer process that yielded transiently the Fe(II) form of the protein. This is the first observation of such a process from a porphyrin/cytochrome self-association complex. Both the rates of bimolecular electron transfer between uncomplexed partners and intramolecular electron transfer from the uroporphyrin triplet to cytochrome c, as well as the thermal intramolecular back-reaction, have been measured by transient kinetic spectroscopy. The rate constants of intramolecular electron transfer for zinc uroporphyrin/cytochrome c and zinc cytochrome c/ferriuroporphyrin have been also determined. These three couples allow us to estimate approximately the reorganization energy λ in the semiclassical electron-transfer theory. © 1990, American Chemical Society. All rights reserved.