PARENT SESSION
Posters P6A Type II reaction centres: Excited state dynamics and donor side. Abstracts (313-346)

Electron transfer in the reaction center of Rb. sphaeroides R-26 studied by transient absorption. Marcin Ziolek, Natalia Wereszczynska, Ryszard Naskrecki, Andrzej Dobek^*,,

ABSTRACT- Electron transfer in the reaction center of the purple photosynthetic bacterium Rb. sphaeroides R-26 was measured at room temperature by use of the time-resolved transient absorption spectroscopy technique. The absorbance changes characteristic of the excited state of the primary donor and extending over the whole spectral range investigated from 350 nm up to 720 nm, appeared within 120 fs delay after excitation with a laser pulse of 80 fs duration at 800 nm. The time evolution of the spectra reflects the excitation of bacteriochlorophylls M and L and subsequent transfer of this excitation to the primary electron donor P, with ₁ time constant, the time of the instrumental function (FWHM=200 fs). From spectra and kinetic analysis the electron transfer reactions to BChl_L and BPhe_L occurring with the fast time constant _f ≤ 4.5 ps, and to ubiquinone Q_A with the slow time constant _s 200 ps. The fast component of time constant, _f, is composed of two time decays, ₂ and ₃. The ₂ time constant describes charge separation between primary donor P and bacteriochlorophyll BChl_L, and ₃ is the time constant of the electron transfer from BChl_L to bacteriopheophytin BPhe_L. Our findings can be interpreted in the following way. The temporal rates ₂ = 3.1 ± 0.5 ps and ₃ = 0.9 ± 0.4 ps support the theoretical model assuming the involvement of the intermediate state P⁺BChl^- in the so-called ′three-step′ model. In this model an electron is transferred in a sequence from the excited special pair P^* to bacteriochlorophyll, BChl_L, then to bacteriopheophytin, BPhe_L, and further on to quinone, Q_A.

KEY WORDS: primary radical pair, purple bacteria, transient absorption, reaction center