title: Vibrational coherence in bacterial reaction centers with genetically modified B-branch pigment composition.
abstract: Femtosecond absorption difference spectroscopy was applied to study the time and spectral evolution of low-temperature (90 K) absorbance changes in isolated reaction centers (RCs) of the HM182L mutant of Rhodobacter (Rb.) sphaeroides. In this mutant, the composition of the B-branch RC cofactors is modified with respect to that of wild-type RCs by replacing the photochemically inactive BB accessory bacteriochlorophyll (BChl) by a photoreducible bacteriopheophytin molecule (referred to as PhiB). We have examined vibrational coherence within the first 400 fs after excitation of the primary electron donor P with 20-fs pulses at 870 nm by studying the kinetics of absorbance changes at 785 nm (PhiB absorption band), 940 nm (P*-stimulated emission), and 1020 nm (BA- absorption band). The results of the femtosecond measurements are compared with those recently reported for native Rb. sphaeroides R-26 RCs containing an intact BB BChl. At delay times longer than approximately 50 fs (maximum at 120 fs), the mutant RCs exhibit a pronounced BChl radical anion (BA-) absorption band at 1020 nm, which is similar to that observed for Rb. sphaeroides R-26 RCs and represents the formation of the intermediate charge-separated state P+ BA-. Femtosecond oscillations are revealed in the kinetics of the absorption development at 1020 nm and of decay of the P*-stimulated emission at 940 nm, with the oscillatory components of both kinetics displaying a generally synchronous behavior. These data are interpreted in terms of coupling of wave packet-like nuclear motions on the potential energy surface of the P* excited state to the primary electron-transfer reaction P*-->P+ BA- in the A-branch of the RC cofactors. At very early delay times (up to 80 fs), the mutant RCs exhibit a weak absorption decrease around 785 nm that is not observed for Rb. sphaeroides R-26 RCs and can be assigned to a transient bleaching of the Qy ground-state absorption band of the PhiB molecule. In the range of 740-795 nm, encompassing the Qy optical transitions of bacteriopheophytins HA, HB, and PhiB, the absorption difference spectra collected for mutant RCs at 30-50 fs resemble the difference spectrum of the P+ PhiB- charge-separated state previously detected for this mutant in the picosecond time domain (E. Katilius, Z. Katiliene, S. Lin, A.K.W. Taguchi, N.W. Woodbury, J. Phys. Chem., B 106 (2002) 1471-1475). The dynamics of bleaching at 785 nm has a non-monotonous character, showing a single peak with a maximum at 40 fs. Based on these observations, the 785-nm bleaching is speculated to reflect reduction of 1% of PhiB in the B-branch within about 40 fs, which is earlier by approximately 80 fs than the reduction process in the A-branch, both being possibly linked to nuclear wave packet motion in the P* state.
authors.affiliation: Department of Photobiophysics, Belozersky Institute of Chemical and Physical Biology, Moscow State University, Moscow 119899, Russian Federation.
authors.forename: Andrei G
authors.initials: AG
authors.lastname: Yakovlev
authors.forename: Tatiana A
authors.initials: TA
authors.lastname: Shkuropatova
authors.forename: Luidmila G
authors.initials: LG
authors.lastname: Vasilieva
authors.forename: Anatoli Ya
authors.initials: AY
authors.lastname: Shkuropatov
authors.forename: Peter
authors.initials: P
authors.lastname: Gast
authors.forename: Vladimir A
authors.initials: VA
authors.lastname: Shuvalov
mesh_terms: D020130:Bacterial Chromatophores / Q000235:genetics / Q000502:physiology*; D001429:Bacteriochlorophylls / Q000235:genetics / Q000502:physiology*; D004579:Electron Transport; D007700:Kinetics; D016297:Mutagenesis, Site-Directed; D010674:Pheophytins / Q000235:genetics / Q000502:physiology*; D045322:Photosynthetic Reaction Center Complex Proteins / Q000235:genetics / Q000502:physiology*; D010860:Pigments, Biological / Q000235:genetics / Q000378:metabolism*; D012242:Rhodobacter sphaeroides / Q000235:genetics / Q000502:physiology*; D013057:Spectrum Analysis
chemical_list: D001429:Bacteriochlorophylls; D010674:Pheophytins; D045322:Photosynthetic Reaction Center Complex Proteins; D010860:Pigments, Biological; C006678:bacteriopheophytin