Temperature dependence of photoinduced electron transfer within self-assembled uroporphyrin-cytochrome c complexes

In this report the temperature dependence of photoinduced electron transfer between both singlet- and triplet-state uroporphyrin ((URO)-U-1 and (URO)-U-3, respectively) and the heme group of ferricytochrome c (Cc) has been examined. Analysis of the forward electron-transfer rate originating from (URO)-U-1 as a function of temperature gives a reorganizational energy (lambda) of 1.0 +/- 0.02 eV with an associated electronic coupling term (H-AB) Of (3.7 +/- 0.3) x 10(-4) eV. In addition, the singlet lifetime data exhibit a temperature-dependent Gaussian distribution that narrows as the temperature increases. This suggests a conformational distribution of bound porphyrin with a narrow range of potential wells at lower temperatures. The corresponding temperature dependence of the electron transfer between (URO)-U-3/URO+ and the heme group of Cc gives a lambda = 0.49 +/- 0.05 eV/0.19 +/- 0.03 eV and H-AB similar to 4 X 10(-4) eV/1.4 x 10(-4) eV, respectively. These observations are consistent with orientational dependent conformational states as well as possible gating of electron transfer within the complex.