Conformational Modulation of Electron Transfer within Electrostatic Porphyrin:  Cytochrome c Complexes

The present study examines photoinduced electron transfer within self-assembled complexes between cytochrome c and either free base uroporphyrin (URO) or free base tetrakis(4-carboxyphenyl)porphyrin (4CP). In both systems, complexation of the porphyrin to the protein results in bathochromic shifts in the absorption bands of the porphyrin. Interestingly, equilibrium circular dichroism data demonstrate significant differences in the orientation of the bound porphyrins. The effect of orientational differences on photoinduced electron transfer between the bound porphyrin and the heme group of cytochrome c are demonstrated in the steady-state and time-resolved fluorescence and triplet−triplet transient absorption data obtained for the two complexes. In the case of the cytochrome c&sbd;4CP complex, the singlet state of the 4CP is significantly quenched by the heme group of the protein. Analysis of the time-resolved fluorescence data reveals two discrete lifetime components at 9.3 (free 4CP) and 1.27 ns (bound 4CP). In contrast, the singlet state of URO is only moderately quenched by complexation to the protein. Fluorescence lifetime analysis reveals two components consisting of a discrete component at 15.7 ns (free URO) and a Lorentzian distribution of lifetimes centered at 3.8 ns. However, URO exhibits significant triplet-state quenching, resulting in intercomplex electron transfer in which the observed forward and reverse rates are similar ((1.8 ± 0.2) × 10⁶ and (1.6 ± 0.4) × 10⁶ s^-1, respectively). The difference in ET mechanism (i.e., singlet versus triplet) can be rationalized in terms of distinct dipole orientations of the bound porphyrins relative to the heme group of the protein. We further speculate that the orientational differences between bound URO and bound 4CP arise due to the flexibility of the URO side chains.