title: B-side charge separation in bacterial photosynthetic reaction centers: nanosecond time scale electron transfer from HB- to QB.
abstract: We report time-resolved optical measurements of the primary electron transfer reactions in Rhodobacter capsulatus reaction centers (RCs) having four mutations: Phe(L181) --> Tyr, Tyr(M208) --> Phe, Leu(M212) --> His, and Trp(M250) --> Val (denoted YFHV). Following direct excitation of the bacteriochlorophyll dimer (P) to its lowest excited singlet state P, electron transfer to the B-side bacteriopheophytin (H(B)) gives P(+)H(B)(-) in approximately 30% yield. When the secondary quinone (Q(B)) site is fully occupied, P(+)H(B)(-) decays with a time constant estimated to be in the range of 1.5-3 ns. In the presence of excess terbutryn, a competitive inhibitor of Q(B) binding, the observed lifetime of P(+)H(B)(-) is noticeably longer and is estimated to be in the range of 4-8 ns. On the basis of these values, the rate constant for P(+)H(B)(-) --> P(+)Q(B)(-) electron transfer is calculated to be between approximately (2 ns)(-)(1) and approximately (12 ns)(-)(1), making it at least an order of magnitude smaller than the rate constant of approximately (200 ps)(-)(1) for electron transfer between the corresponding A-side cofactors (P(+)H(A)(-) --> P(+)Q(A)(-)). Structural and energetic factors associated with electron transfer to Q(B) compared to Q(A) are discussed. Comparison of the P(+)H(B)(-) lifetimes in the presence and absence of terbutryn indicates that the ultimate (i.e., quantum) yield of P(+)Q(B)(-) formation relative to P is 10-25% in the YFHV RC.
authors.affiliation: Department of Chemistry, Washington University, St. Louis, Missouri 63130, USA.
authors.forename: Christine
authors.initials: C
authors.lastname: Kirmaier
authors.forename: Philip D
authors.initials: PD
authors.lastname: Laible
authors.forename: Deborah K
authors.initials: DK
authors.lastname: Hanson
authors.forename: Dewey
authors.initials: D
authors.lastname: Holten
mesh_terms: D001429:Bacteriochlorophylls / Q000737:chemistry*; D016227:Benzoquinones / Q000737:chemistry*; D019281:Dimerization; D004579:Electron Transport / Q000235:genetics; D006639:Histidine / Q000235:genetics; D007096:Imidoesters / Q000737:chemistry; D007700:Kinetics; D045342:Light-Harvesting Protein Complexes; D009154:Mutation; D036103:Nanotechnology / Q000379:methods; D010649:Phenylalanine / Q000235:genetics; D010674:Pheophytins / Q000737:chemistry*; D010782:Photolysis; D045322:Photosynthetic Reaction Center Complex Proteins / Q000737:chemistry* / Q000235:genetics; D016354:Rhodobacter capsulatus / Q000737:chemistry* / Q000235:genetics; D055672:Static Electricity; D014227:Triazines / Q000737:chemistry; D014443:Tyrosine / Q000235:genetics
chemical_list: D001429:Bacteriochlorophylls; D016227:Benzoquinones; D007096:Imidoesters; D045342:Light-Harvesting Protein Complexes; D010674:Pheophytins; D045322:Photosynthetic Reaction Center Complex Proteins; D014227:Triazines; C034266:N-lauryl iminodipropionate; C006678:bacteriopheophytin; C004532:quinone; D014443:Tyrosine; D010649:Phenylalanine; D006639:Histidine; C010347:terbutryne