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

The absorption changes of chlorophyll (Chi) 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 (t1/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 Ao~, was detected after the laser excitation in addition to the shift at 668 nm indicating the formation of the P840+A0~ state. The P840+Ao" state decayed with a t 1/e of 43 ns at 77 K and produced a triplet state P840T 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 Ao, 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.