2Department of Biophysics, Huygens Laboratory, Leiden University, P. O. Box 9504, 2300 RA Leiden, The Netherlands
3Institute of Basic Biological Problems, Russian Academy of Sciences, 142290 Pushchino, Moscow Region, Russia; fax: (496) 773-0532; E-mail: email@example.com
* To whom correspondence should be addressed.
Received November 19, 2009
Coherent processes in an initial phase of charge transfer in reaction centers (RCs) of the triple mutant S(L178)K/G(M203)D/L(M214)H of Rhodobacter sphaeroides were investigated by difference (light – dark) absorption spectroscopy with 18 fsec time resolution. Electron transfer in the B cofactor branch is activated in this mutant, while the A-branch electron transfer is slowed in comparison with native RCs of Rba. sphaeroides. A bulk of absorption difference spectra was analyzed in the 940-1060 nm range (stimulated emission of excited bacteriochlorophyll dimer P* and absorption of bacteriochlorophyll anions BA– and β–, where β is a bacteriochlorophyll substituting the native bacteriopheophytin HA) and in the 735-775 nm range (bleaching of the absorption band of the bacteriopheophytin HB in the B-branch) in the –0.1 to 4 psec range of delays with respect to the moment of photoexcitation of P at 870 nm. Spectra were measured at 293 and 90 K. The kinetics of P* stimulated emission at 940 nm shows its decay with a time constant of ~14 psec at 90 K and ~18 psec at 293 K, which is accompanied by oscillations with a frequency of ~150 cm–1. A weak absorption band is found at 1018 nm that is formed ~100 fsec after excitation of P and reflects the electron transfer from P* to β and/or BA with accumulation of the P+β– and/or P+BA– states. The kinetics of ΔA at 1018 nm contains the oscillations at ~150 cm–1 and distinct low-frequency oscillations at 20-100 cm–1; also, the amplitude of the oscillations at 150 cm–1 is much smaller at 293 than at 90 K. The oscillations in the kinetics of the 1018 nm band do not contain a 32 cm–1 mode that is characteristic for native Rba. sphaeroides RCs having water molecule HOH55 in their structure. The ΔA kinetics at 751 nm reflects the electron transfer to HB with formation of the P+HB– state. The oscillatory part of this kinetics has the form of a single peak with a maximum at ~50 fsec completely decaying at ~200 fsec, which might reflect a reversible electron transfer to the B-branch. The results are analyzed in terms of coherent nuclear wave packet motion induced in the P* excited state by femtosecond light pulses, of an influence of the incorporated mutations on the mutual position of the energy levels of charge separated states, and of the role of water HOH55 in the dynamics of the initial electron transfer.
KEY WORDS: photosynthesis, charge separation, reaction center, wave packet, electron transfer