FTIR SPECTROSCOPY OF PRIMARY DONOR PHOTOOXIDATION IN PHOTOSYSTEM-I, HELIOBACILLUS MOBILIS, AND CHLOROBIUM-LIMICOLA - COMPARISON WITH PURPLEBACTERIA

The photooxidation of the primary electron donor in several Photosyste m I-related organisms (Synechocystis sp. PCC 6803, Heliobacillus mobil is, and Chlorobium limicola f. sp. thiosulphatophilum) has been studie d by light-induced FTIR difference spectroscopy at 100 K in the 4000 t o 1200 cm(-1) spectral range. The data are compared to the well-charac terized FTIR difference spectra of the photooxidation of the primary d onor P in Rhodobacter sphaeroides (both wild type and the heterodimer mutant HL M202) in order to get information on the charge localization and the extent of coupling within the (bacterio)chlorophylls constitu ting the oxidized primary donors. In Rb. sphaeroides RC, four marker b ands mostly related to the dimeric nature of the oxidized primary dono r have been previously observed at approximate to 2600, 1550, 1480, an d 1295 cm(-1). The high-frequency band has been shown to correspond to an electronic transition (Breton et al. (1992) Biochemistry 31: 7503- 7510) while the three other marker bands have been described as phase- phonon bands (Reimers and Hush (1995) Chem Phys 197: 323-332). The abs ence of these bands in PS I as well as in the heterodimer HL M202 demo nstrates that in P700(+) the charge is essentially localized on a sing le chlorophyll molecule. For both H. mobilis and C. limicola, the pres ence of a high-frequency band at approximate to 2050 and 2450 cm(-1), respectively, and of phase-phonon bands (at approximate to 1535 and 13 00 cm(-1) in H. mobilis, at approximate to 1465 and 1280 cm(-1) in C. limicola) indicate that the positive charge in the photooxidized prima ry donor is shared between two coupled BChls. The structure of P840(+) in C. limicola, in terms of the resonance interactions between the tw o BChl a molecules constituting the oxidized primary donor, is close t o that of P+ in purple bacteria reaction centers while for H. mobilis the FTIR data are interpreted in terms of a weaker coupling of the two bacteriochlorophylls.