The electron transfer rate from BPh_A to Q_A in reaction centers of Rhodobacter sphaeroides R-26: Influence of the H-subunit, the Q_A and Fe²⁺ cofactors, and the isoprene tail of Q_A

The secondary electron transfer from reduced bacteriopheophytin (BPh_A) to the first acceptor quinone (Q_A) in variously modified reaction centers from Rhodobacter sphaeroides was studied by transient absorption spectroscopy and compared with native reaction centers. In intact reaction centers, neither substitution of the native Q_A, ubiquinone-10(UQ₁₀), with menaquinone (MK), nor shortening of the isoprene tail of ubiquinone (UQ) down to 4 and that of MK down to 2 isoprene units changed the rate of electron transfer to Q_A. However, in Fe²⁺-depleted reaction centers the electron transfer rate decreased by a factor of 5 upon MK-reconstitution and by a factor of 20-50 upon non-native UQ-reconstitution. In the latter particles, the electron transfer rate decreased with decreasing tail length of UQ, suggesting that the displacement of UQ within the Q_A pocket as proposed in previous work (Liu B.-L., Van Kan P.J.M. and Hoff A.J. (1991) FEBS Lett. 289, 23-28), is tail-length-dependent.