Evidence that a distribution of bacterial reaction centers underlies the temperature and detection-wavelength dependence of the rates of the primary electron-transfer reactions

The rates of the primary electron-transfer processes in Rhodobacter sphaeroides reaction centers have been examined in detail by using 150-fs excitation flashes at 870 nm. At room temperature the apparent time constants for both initial charge separation (P* → P⁺BPh_L^-) and subsequent electron transfer (P⁺BPh_L^- → P⁺Q_A^-) are found to encompass a range of values (≈1.3-4 ps and ≈100-320 ps, respectively), depending on the wavelength at which the kinetics are followed. We suggest this reflects a distribution of reaction centers (or a few conformers), having differences in factors such as distances or orientations between the cofactors, hydrogen bonding, or other pigment-protein interactions. We also suggest that the time constants observed at cryogenic temperatures (≈1.3 and ≈100 ps, respectively, with much smaller or negligible variation with detection wavelength) do not reflect an actual increase in the rates with decreasing temperature but rather derive from a shift in the distribution of reaction centers toward those in which electron transfer inherently occurs with the faster rates.