title: Determination of the rate and yield of B-side quinone reduction in Rhodobacter capsulatus reaction centers.
abstract: In the native purple bacterial reaction center (RC), light-driven charge separation utilizes only the A-side cofactors, with the symmetry related B-side inactive. The process is initiated by electron transfer from the excited primary donor (P*) to the A-side bacteriopheophytin (P* --> P+ H(A)-) in approximately 3 ps. This is followed by electron transfer to the A-side quinone (P+ H(A)- --> P+ Q(A)-) in approximately 200 ps, with an overall quantum yield of approximately 100%. Using nanosecond flash photolysis and RCs from the Rhodobacter capsulatus F(L181)Y/Y(M208)F/L(M212)H mutant (designated YFH), we have probed the decay pathways of the analogous B-side state P+ H(B)-. The rate of the P+ H(B)- --> ground-state charge-recombination process is found to be (3.0 +/- 0.8 ns)(-1), which is much faster than the analogous (10-20 ns)(-1) rate of P+ H(A)- --> ground state. The rate of P+ H(B)- --> P+ Q(B)- electron transfer is determined to be (3.9 +/- 0.9 ns)(-1), which is about a factor of 20 slower than the analogous A-side process P+ H(A)- --> P+ Q(A)-. The yield of P+ H(B)- --> P+ Q(B)- electron-transfer calculated from these rate constants is 44%. This value, when combined with the known 30% yield of P+ H(B)- from P in YFH RCs, gives an overall yield of 13% for B-side charge separation P* --> P+ H(B)- --> P+ Q(B)- in this mutant. We determine essentially the same value (15%) by comparing the P-bleaching amplitude at approximately 1 ms in YFH and wild-type RCs.
authors.affiliation: Department of Chemistry, Washington University, St. Louis, Missouri 63130, USA.
authors.forename: Hooi Ling
authors.initials: HL
authors.lastname: Kee
authors.forename: Philip D
authors.initials: PD
authors.lastname: Laible
authors.forename: James A
authors.initials: JA
authors.lastname: Bautista
authors.forename: Deborah K
authors.initials: DK
authors.lastname: Hanson
authors.forename: Dewey
authors.initials: D
authors.lastname: Holten
authors.forename: Christine
authors.initials: C
authors.lastname: Kirmaier
mesh_terms: D007700:Kinetics; D010084:Oxidation-Reduction; D045322:Photosynthetic Reaction Center Complex Proteins / Q000378:metabolism*; D011809:Quinones / Q000378:metabolism*; D016354:Rhodobacter capsulatus / Q000378:metabolism*
chemical_list: D045322:Photosynthetic Reaction Center Complex Proteins; D011809:Quinones