-deltaG(AB) and pH dependence of the electron transfer from P(+)Q(A)(-)Q(B) toP(+)Q(A)Q(B)(-) in Rhodobacter sphaeroides reaction centers.

The electron transfer from the reduced primary quinone (QA - ) to the secondary quinone (QB) can occur in two phases with a well-characterized 100 Is component (U2) and a faster process occurring in less than 10 I s( U1). The fast reaction is clearly seen when the native ubiquinone-10 at Q A is replaced with naphthoquinones. The dependence of U1 on the free-energy difference between the P + QA - QB and P + QAQB - states (-¢G°) and on the pH was measured using naphthoquinones with different electrochemical midpoint potentials as QA in Rhodobacter sphaeroidesreaction centers (RCs) and in RCs where -¢G° is changed by mutation of M265 in the QA site from Ile to Thr (M265IT). QB was ubiquinone (UQB) in all cases. Electron transfer was measured by using the absorption differences of the naphthosemiquinone at QA and the ubisemiquinone at QB between 390 and 500 nm. As -¢G° was changed from -90 to -250 meV U1 decreased from 29 to 0.2 Is. The free-energy dependence of U1 provides a reorganization energy of 850 ( 100 meV for the electron transfer from QA - to QB. The slower reaction at U2 is free-energy independent, so processes other than electron transfer determine the observed rate. The fraction of the reaction at U1 increases with increasing driving force and is 100% of the reaction when -¢G° is 100 meV more favorable than in the native RCs with ubiquinone as QA. The fast phase, U1, is pH independent from pH 6 to 11 while U2 slows above pH 9. As the QA isoprene tail length is increased from 2 to 10 isoprene units the fraction at U1 decreases. However, U1, U2, and the fraction of the reaction in each phase are independent of the tail length of UQB.