title: Kinetic phases in the electron transfer from P+QA-QB to P+QAQB- and the associated processes in Rhodobacter sphaeroides R-26 reaction centers.
abstract: Electron transfer from P+QA-QB to form P+QAQB- was measured in Rhodobacter sphaeroides R-26 reaction centers (RCs) where the native primary quinone, ubiquinone-10 (UQA), was replaced by 2-methyl-3-phytyl-1,4-naphthoquinone (MQA). The native secondary quinone, UQ-10, was retained as UQB. The difference spectrum of the semiquinone MQA- minus UQB- 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 MQA- to UQB. In contrast, when both QA and QB are UQ-10 the spectral difference between UQA- and UQB- 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 tau1 approximately 3 micros are mostly due to electron transfer, electron transfer and charge compensation are mixed in tau2 approximately 80 micros, while little or no electron transfer occurs at 200-600 micros (tau3) in MQAUQB RCs. The 80-micros 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 DeltaG approximately 3.5 kcal/mol for both tau1 and tau2 between 0 and 30 degrees C. In isolated RCs with UQA, if there is any fast component, it appears to be faster and less important than in the MQA reconstituted RCs.
authors.affiliation: Department of Physics, Room J419, City College of New York, 138th Street and Convent Avenue, New York, New York 10031, USA.
authors.forename: J
authors.initials: J
authors.lastname: Li
authors.forename: D
authors.initials: D
authors.lastname: Gilroy
authors.forename: D M
authors.initials: DM
authors.lastname: Tiede
authors.forename: M R
authors.initials: MR
authors.lastname: Gunner
mesh_terms: D004579:Electron Transport; D007700:Kinetics; D008956:Models, Chemical; D010674:Pheophytins / Q000378:metabolism; D045322:Photosynthetic Reaction Center Complex Proteins / Q000378:metabolism*; D012242:Rhodobacter sphaeroides / Q000378:metabolism*; D014451:Ubiquinone / Q000378:metabolism; D014812:Vitamin K / Q000378:metabolism; D010837:Vitamin K 1 / Q000378:metabolism
chemical_list: D010674:Pheophytins; D045322:Photosynthetic Reaction Center Complex Proteins; D014812:Vitamin K; D014451:Ubiquinone; C006678:bacteriopheophytin; D010837:Vitamin K 1; C025204:Ubiquinone Q2