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.