title: Transient states in reaction centers containing reduced bacteriopheophytin.
abstract: Photosynthetic reaction centers isolated from Rhodopseudomonas sphaeroides strain R-26 were excited with non-saturating 7-ps, 600-nm flashes under various conditions, and the resulting absorbance changes were measured. If the quinone electron acceptor (Q) is in the oxidized state, flash excitation generates a transient state (PF), in which an electron has moved from the primary electron donor (P, a dimer of bacteriochlorophylls) to an acceptor complex involving a special bacteriopheophytin (H) and another bacteriochlorophyll (B). PF decays in 200 ps as an electron moves from H to Q and the acceptor complex are reduced photochemically before the excitation, the flash generates a different transient state of P with a high quantum yield. This state decays with a lifetime of 340 ps. There is no indication of electron transfer from P to B under these conditions, but this does not rule out the possibility that B is an intermediate electron carrier between P and H. Measurements of the yield of fluorescence from P under various conditions show that the 340 ps state is not the fluorescent excited singlet state of P. The transient state could be a triplet state, a charge-transfer state of P, or another excited singlet state that is not fluorescent.
mesh_terms: D000332:Aerobiosis; D002734:Chlorophyll / Q000031:analogs & derivatives*; D003624:Darkness; D004579:Electron Transport; D007700:Kinetics; D008027:Light; D010674:Pheophytins / Q000378:metabolism*; D010788:Photosynthesis; D011789:Quantum Theory; D012242:Rhodobacter sphaeroides / Q000378:metabolism*; D013053:Spectrophotometry
chemical_list: D010674:Pheophytins; D002734:Chlorophyll; C006678:bacteriopheophytin