Conformational Dynamics and Temperature Dependence of Photoinduced Electron Transfer within Self-Assembled Coproporphyrin:Cytochrome c Complexes

Abstract

The focus of the present study is to better understand the complex factors influencing intermolecular electron transfer (ET) in biological molecules using a model system involving free-base coproporphyrin (COP) complexed with horse heart cytochrome c (Cc). Coproporphyrin exhibits bathochromic shifts in both the Soret and visible absorption bands in the presence of Cc and an absorption difference titration reveals a 1:1 complex with an association constant of 2.63 ± 0.05 × 10 5 M −1. At 20°C, analysis of time-resolved fluorescence data reveals two lifetime components consisting of a discrete lifetime at 15.0 ns (free COP) and a Gaussian distribution of lifetimes centered at 2.8 ns (representing 1COP → Cc ET). Temperature-dependent, time-resolved fluorescence data demonstrate a shift in singlet lifetime as well as changes in the distribution width (associated with the complex). By fitting these data to semiclassical Marcus theory, the reorganizational energy ( λ) of the singlet state electron transfer was calculated to be 0.89 eV, consistent with values for other porphyrin/Cc intermolecular ET reactions. Using nanosecond transient absorption spectroscopy the temperature dependences of the forward and thermal back ET originating from triplet state were examined ( 3COP → Cc ET). Fits of the temperature dependence of the rate constants to semiclassical Marcus theory gave λ of 0.39 eV and 0.11 eV for the forward and back triplet ET, respectively ( k f = (7.6 ± 0.3) × 10 6 s −1, k b = (2.4 ± 0.3) × 10 5 s −1). The differing values of λ for the forward and back triplet ET demonstrate that these ET reactions do not occur within a static complex. Comparing these results with previous studies of the uroporphyrin:Cc and tetrakis (4-carboxyphenyl)porphyrin:Cc complexes suggests that side-chain flexibility gives rise to the conformational distributions in the 1COP → Cc ET whereas differences in overall porphyrin charge regulates gating of the back ET reaction (reduced Cc → COP +).