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 (Chl) a-like pigments (C670) were stu died by ns-ms laser spectroscopy at 77 K in the untreated and urea-treated homodimeric reaction center (RC) complex of the green sulfur bacterium Chlo robium tepidum. The untreated RC complex contained 9 molecules of C670 in a ddition to 41 molecules of Bchl a and 0.9 molecules of menaquinone-7 per on e primary electron donor Bchl a dimer (P840), Upon photo-oxidation of P840, C670 showed an absorption change of a red-shift with an isosbestic wavelen gth 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 re present the charge recombination between P840(+) and FeS-. In the urea-trea ted 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 d etected after the laser excitation in addition to the shift at 668 nm indic ating the formation of the P840(+)A(0)(-) state. The P840(+)A(0)(-) state d ecayed 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 indi cate the two different types of C670 species in the RC complex; the one pea king at 670 nm functions as A(0), while the other peaking at 668 nm shows t he electrochromic shift, which presumably functions as the accessory pigmen t located in the close vicinity of P840.