Time-Resolved Spectroscopy of Chlorophyll-a Like Electron Acceptor in the Reaction Center Complex of the Green Sulfur Bacterium Chlorobium tepidum

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The absorption changes of chlorophyll (Chl) a-like pigments (C670) were studied by ns-ms laser spectroscopy at 77 K in the untreated and urea-treated homodimeric reaction center (RC) complex of the green sulfur bacterium Chlorobium tepidum. The untreated RC complex contained 9 molecules of C670 in addition to 41 molecules of Bchl a and 0.9 molecules of menaquinone-7 per one primary electron donor Bchl a dimer (P840). Upon photo-oxidation of P840, C670 showed an absorption change of a red-shift with an isosbestic wavelength at 668 nm. The absorption change of P840 decayed with time constants (t_<1/e>) of 55 and 37 ms at 283 and 77 K, respectively, and was assigned to represent the charge recombination between P840^+ and FeS^-. In the urea-treated RC complex, a bleach peaking at 670 nm with a shoulder peak at 662 nm, which is ascribable to the reduced primary electron acceptor A_0^-, was detected after the laser excitation in addition to the shift at 668 nm indicating the formation of the P840^+A_0^- state. The P840^+A_0^- state decayed with a t_<1/e> of 43 ns at 77 K and produced a triplet state P840^T due to the suppression of the forward electron transfer. These results indicate the two different types of C670 species in the RC complex; the one peaking at 670 nm functions as A_0, while the other peaking at 668 nm shows the electrochromic shift, which presumably functions as the accessory pigment located in the close vicinity of P840.