The primary electron transfer in reaction centers ofRhodobacter sphaeroides is studied by subpicosecond absorption spectroscopy with polarized light in the spectral range of 920-1040 nm. Here the bacteriochlorophyll anion radical has an absorption band while the other pigments of the reaction center have vanishing ground-state absorption. The transient absorption data exhibit a pronounced 0.9-ps kinetic component which shows a strong dichroism. Evaluation of the data yields an angle between the transition moments of the special pair and the species related with the 0.9-ps kinetic component of 26 ± 8. This angle compares favorably with the value of 29°expected for the reduced accessory bacteriochlorophyll. Extensive transient absorbance data are fufly consistent with a stepwise electron ransfer via the accessory bacteriochlorophyll.In the primary processes of bacterial photosynthesis, absorbed light energy is stored via an electron transfer within the reaction center (RC). While the molecular structure of two bacterial reaction centers has been known for a number of years (1-3), the detailed molecular mechanism of the electron transfer is still the subject of intense investigations (4-20). There is general agreement that the first electron transfer process starts at a pair of bacteriochlorophyll (BChl) molecules-the special pair P-which acts as the primary donor. The

ABSTRACT:Femtosecond spectroscopy was used in combination with site-directed mutagenesis to study the influence of tyrosine M210 (YM210) on the primary electron transfer in the reaction center of Rhodobacter sphaeroides. The exchange of YM210 to phenylalanine caused the time constant of primary electron transfer to increase from 3.5 f 0.4 ps to 16 f 6 ps while the exchange to leucine increased the time constant even more to 22 f 8 ps. The results suggest that tyrosine M210 is important for the fast rate of the primary electron transfer.x e primary photochemical event during photosynthesis of bacteriochlorophyll-(Bchl-) containing organisms is a lightinduced charge separation within a transmembrane protein complex called the reaction center (RC). The crystal structures of RC's from Rhodopseudomonas (Rps.) viridis and Rhodobacter (Rb.) sphaeroides have been solved to high resolution [reviewed in Deisenhofer and Michel (1989), Chang et al. (1986), Tiede et al. (1988), andRees et al. (1989)l. The RC from Rb. sphaeroides contains three protein subunits referred to as L, M, and H, according to their respective mobilities in SDS-polyacrylamide gels. Associated with the L and M subunits are the cofactors, consisting of four Bchl a, two bacteriopheophytin (Bph) a, one atom of non-heme ferrous iron, two quinones (QA and Qe), and in some species one carotenoid [reviewed in Parson (1987) and Feher et al.' Financial support was from the Deutsche Forschungsgemeinschaft,

The resonance-enhanced multiphoton ionization (REMPI) of the OH radical in the gas phase was studied using an isothermal discharge-flow reactor for the production of OH radicals (H+NO,+OH+NO), tunable laser light from an excimer-pumpeddye laser, and a time-of-flight (TOF) mass spectrometer. A mass-resolved REMPI spectrum was found in the wavelength region of 290-3 10 nm, which is assigned to a (3 + I ) ionization process. An ab initio quantum-chemical calculation predicts the Rydberg state, verified by the experiment.

The primary photosynthetic reactions in whole membranes of the antenna-deficient mutant strain U43 (pTXA6-10) of Rhodobacter capsulatus are studied by transient absorption and emission spectroscopy with subpicosecond time resolution. Extensive similarities between the transient absorption data on whole membranes and on isolated reaction centers support the idea that the primary processes in isolated reaction centers are not modified by the isolation procedure.

M subunit Trp252 is the only amino acid residue which is located between the bacteriopheophytin HA and the quinone QA in the photosynthetic reaction centre of Rhodobacter sphaeroides. Oligodeoxynucleotide-directed mutagenesis was employed to elucidate the influence of this aromatic amino acid on the electron transfer between these two chromophores. For this, M subunit Trp252 was changed to tyrosine or phenylalanine, and Thr222, which presumably forms a hydrogen bridge to the indole ring of M subunit Trp252, to valine. In all three mutated reaction centres, the electronaccepting ubiquinone QA is less firmly bound to its binding site than in the wild-type protein. The electron transfer from the reduced bacteriopheophytin HA-to Q A proceeds in the wild-type and in the mutant ThrM222Val within 220ps. However, in the mutants TrpM252Tyr and TrpM252Phe the time constants are 600 ps and 900 ps, respectively. This indicates that M subunit Trp252 participates in the binding of QA and reduction of this quinone.