PARENT SESSION
Posters P2A Type I reaction centres. Abstracts (181-218)

Energy transfer and charge separation dynamics in chlorophyll d-based Photosystem I from Acaryochloris marina. Shigeichi Kumazaki^*,1, Isamu Ikegami², Shigeru Itoh³, ¹ Department of Chemistry, Graduate School of Science, Kyoto, Japan² Faculty of Pharmaceutical Sciences, Sagamiko, Kanagawa, Japan³ Department of Physics, Graduate School of Science, Nagoya, Japan

ABSTRACT- This paper reports the primary photochemistry on a (sub)picosecond time scale in the purified photosystem I (PSI) from Acaryochloris marina, which adopts chlorophyll d-absorbing at 700 - 740 nm as its major chlorophyll. The dynamical features are compared with those of chlorophyll a-based PSI. The electron transfer dynamics was clarified by observing the dependence of transient absorption spectra of photosystem I on the redox conditions of the electron donor Chls (P740 or P700) in the ground state. This is enabled by the quenching of Chl excited states by both reduced and oxidized electron donor Chl, only the former of which leads to the primary charge-separated state. The energy transfer dynamics was revealed by observing the excitation-wavelength dependence of the transient absorbance spectra. This is based on the fact that chlorophylls in PSI show characteristic absorption peaks depending on their binding sites, which leads to different initial distributions of excitation energy for different excitation wavelengths. We have found that an intermeadite charge-separated state before the reduction of quinone is far more accumulated in chlorophyll d-based PSI than in chlorophyll a-based PSI, which suggests a relatively fast phase of trapping of the excitation energy (about 7 ps) and a slow reduction of acceptor quinone (about 50 ps) in the case of former PSI. This overall feature of the charge separation was found to be insensitive to excitation wavelength. The time required for spectral equilibration of the transient absorption spectra in the case of chlorophyll d-based PSI (about 5 ps) was found to be similar to those in the case of the core complex of chlorophyll a-based PSI, which is attributable to similarity in the overall arrangement of antenna chlorophylls.

KEY WORDS: charge separation, energy transfer, reaction center, chlorophyll d