Direct charge recombination from D+QAQB− to DQAQB in bacterial reaction centers from Rhodobacter sphaeroides containing low potential quinone in the QA site Original Research Article

In native RCs from Rb. sphaeroides the recombination D+QAQB− → DQAQB proceeds via an indirect path involving the intermediate state D+QA−QB. To observe the direct recombination rate, kBD, the energy difference between the D+QA−QB and D+QAQB− states has to be increased. This had been accomplished in mutant RCs (DN(L213)) by lowering the energy of the D+QAQB− state [A. Labahn, M.L. Paddock, P.H. McPherson, M.Y. Okamura and G. Feher, J. Phys. Chem. 98 (1994) 3417] or, as presented in this work, by arising the energy of the D+QA−QB state through substitution of Q10 by the low potential quinones: (2,3,5-trimethyl-1,4-naphthoquinone, 2,3,6,7-tetramethyl-1,4-naphthoquinone, 2-chloro-9,10-anthraquinone) while retaining the native Q10 in the QB site. The recombination rates kBD in these hybrid RCs were fitted with the Marcus theory giving a reorganization energy, λBD = 1.1 ± 0.1 eV and an electronic matrix element V(r) = (1.2 ± 0.5) × 10−8 eV. The larger value of λBD compared to λAD (1.1 versus 0.6 eV) is consistent with the more polar environment of QB− and is believed to be the main contributor to the large observed ratio of kAD/kBD ≈ 100.