Energetics of initial charge separation in bacterial photosynthesis: The triplet decay rate in very high magnetic fields

Abstract

The triplet state of the primary electron donor is formed in bacterial reaction centers when electron transfer to the quinone is blocked. The relative quantum yield of formation and the rate of decay of the triplet state were measured in quinone-depleted reaction centers from Rhodobacter sphaeroides as a function of temperature and magnetic field up to 135 kG. By analyzing these dependences we obtain a standard free-energy difference at room temperature between the radical pair intermediate and the triplet state of 1370 ± 30 cm −1 (0.170 ± 0.004 eV), and a standard enthalpy difference of 1450 ± 70 cm −1 (0.180 ± 0.009 eV). These results differ substantially from those obtained previously using lower fields. This difference is ascribed primarily to effects of nuclear spin polarization. In combination with the known energies of the excited singlet and triplet states of the primary electron donor, we calculate that the standard free-energy change for the initial charge separation reaction is 2120 cm −1 (0.263 eV) and that the entropy change is small.