Generation of ion-radical chlorophyll states in the light-harvesting antenna and the reaction center of cyanobacterial photosystem I

The energy and charge-transfer processes in photosystem I (PS I) complexes isolated from cyanobacteriaThermosynechococcus elongatusandSynechocystissp. PCC 6803 were investigated by pump-to-probe femtosecond spectroscopy. The formation of charge-transfer (CT) states in excitonically coupled chlorophyllacomplexes (exciplexes) was monitored by measuring the electrochromic shift of beta-carotene in the spectral range 500-510 nm. The excitation of high-energy chlorophyll in light-harvesting antenna of both species was not accompanied by immediate appearance of an electrochromic shift. In PS I fromT. elongatus, the excitation of long-wavelength chlorophyll (LWC) caused a pronounced electrochromic effect at 502 nm assigned to the appearance of CT states of chlorophyll exciplexes. The formation of ion-radical pair P(700)(+)A(1)(-)at 40 ps was limited by energy transfer from LWC to the primary donor P(700)and accompanied by carotenoid bleach at 498 nm. In PS I fromSynechocystis6803, the excitation at 720 nm produced an immediate bidentate bleach at 690/704 nm and synchronous carotenoid response at 508 nm. The bidentate bleach was assigned to the formation of primary ion-radical state P(B)(+)Chl(2B)(-), where negative charge is localized predominantly at the accessory chlorophyll molecule in the branch B, Chl(2B). The following decrease of carotenoid signal at similar to 5 ps was ascribed to electron transfer to the more distant molecule Chl(3B). The reduction of phylloquinone in the sites A(1A)and A(1B)was accompanied by a synchronous blue-shift of the carotenoid response to 498 nm, pointing to fast redistribution of unpaired electron between two branches in favor of the state P(B)(+)A(1A)(-).