Kinetic phases in the electron transfer from P+QA-QB to P+QAQB- and the associated processes in Rhodobacter sphaeroides R-26 reaction centers

Electron transfer from p(+)Q(A)(-)Q(B) to form p(+)Q(A)Q(B)(-) was measured in Rhodobacter sphaeroides R-26 reaction centers (RCs) where the native primary quinone, ubiquinone-10 (UQ(A)), was replaced by 2-methyl-3-phytyl-1,4-naphthoquinone (MQ(A)) The native secondary quinone, UQ-10, was retained as UQ(B) The difference spectrum of the semiquinone MQ(A)(-) minus UQ(B)(-) absorption is very similar to that of MQ(-) minus UQ(-) in solution (398-480 nm). Thus, the absorption change provides a direct monitor of the electron transfer from MQ(A)(-) to UQ(B) In contrast, when both Q(A) and Q(B) are UQ-10 the spectral difference between UQ(A)(-) and UQ(B)(-) arises from electrochromic responses of RC chromophores. Three kinetic processes are seen in the near UV (390-480 nm) and near-IR (740-820 nm). Analysis of the time-correlated spectra support the conclusion that the changes at tau(1) approximate to 3 mu s are mostly due to electron transfer, electron transfer and charge compensation are mixed in tau(2) approximate to 80 mu s, while little or no electron transfer occurs at 200-600 mu s (tau(3)) in MQ(A)UQ(B) RCs. The 80-mu s rate has been previously observed, while the fast component has not. The fast phase represents 60% of the electron-transfer reaction (398 nm). The activation energy for electron transfer is Delta G approximate to 3.5 kcal/mol for bath tau(1) and tau(2) between 0 and 30 degrees C. In isolated RCs with UQ(A), if there is any fast component, it appears to be faster and less important than in the MQ(A) reconstituted RCs.