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*rightarrow P+BPh- l) and subsequent electron transfer (P+BPh- Lrightarrow 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.