Pathways of energy transformation in antenna reaction center complexes of Heliobacillus mobilis

The conversion of excitation energy in the antenna reaction center complex of Heliobacillus mobilis was investigated at 10 K as well as at 275 K by me ans of time-resolved absorbance difference spectroscopy of isolated membran es in the (sub)picosecond time range. Selective excitation of the primary e lectron acceptor, chlorophyll (Chl) a 670, and of the different spectral po ols of bacteriochlorophyll (BChl) g (BChl g 778, BChl g 793, and BChl g 808 ) was applied. At 10 K, excitation at 770 or 793 nm resulted on the one han d in rapid energy transfer to BChl g 808 and on the other hand in fast char ge separation from excited BChl g 793 (similar to 1 ps). Once the excitatio ns were on BChl g 808, the bleaching band shifted gradually to the red, fro m 806 to 813 nm, and charge separation from excited BChl g 808 occurred by a very slow process (similar to 500 ps). The main purpose of our experiment s was to answer the question whether an "alternative" pathway for charge se paration exists upon excitation of Chi a 670. Our measurements showed that the amount of oxidized primary donor (P798(+)) relative to that of excited BChl g produced by excitation of Chi cr 670 was considerably larger than up on direct excitation of BChl g. This indicates the existence of an alternat ive pathway for charge separation that does not involve excited antenna BCh l g. This effect occurred at 10 K as well as at 275 K. The mechanism for th is process is discussed in relation to different trapping models; it is con cluded that charge separation occurs directly from excited Chi a 670.