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 means of time-resolved absorbance difference spectroscopy of isolated membranes in the (sub)picosecond time range. Selective excitation of the primary electron acceptor, chlorophyll (Chl) a 670, and of the different spectral pools 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 hand in rapid energy transfer to BChl g 808 and on the other hand in fast charge separation from excited BChl g 793 ( approximately 1 ps). Once the excitations were on BChl g 808, the bleaching band shifted gradually to the red, from 806 to 813 nm, and charge separation from excited BChl g 808 occurred by a very slow process ( approximately 500 ps). The main purpose of our experiments was to answer the question whether an "alternative" pathway for charge separation exists upon excitation of Chl a 670. Our measurements showed that the amount of oxidized primary donor (P798(+)) relative to that of excited BChl g produced by excitation of Chl a 670 was considerably larger than upon direct excitation of BChl g. This indicates the existence of an alternative pathway for charge separation that does not involve excited antenna BChl g. This effect occurred at 10 K as well as at 275 K. The mechanism for this process is discussed in relation to different trapping models; it is concluded that charge separation occurs directly from excited Chl a 670.