FTIR spectroscopy of primary donor photooxidation in Photosystem I, Heliobacillus mobilis, and Chlorobium limicola. Comparison with purple bacteria

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Abstract
The photooxidation of the primary electron donor in several Photosystem I-related organisms (Synechocystis sp. PCC 6803, Heliobacillus mobilis, and Chlorobium limicola f. sp. thiosulphatophilum) has been studied by light-induced FTIR difference spectroscopy at 100 K in the 4000 to 1200 cm−1 spectral range. The data are compared to the well-characterized FTIR difference spectra of the photooxidation of the primary donor 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 constituting the oxidized primary donors. In Rb. sphaeroides RC, four marker bands mostly related to the dimeric nature of the oxidized primary donor have been previously observed at ≈2600, 1550, 1480, and 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 absence of these bands in PS I as well as in the heterodimer HL M202 demonstrates that in P700+ the charge is essentially localized on a single chlorophyll molecule. For both H. mobilis and C. limicola, the presence of a high-frequency band at ≈ 2050 and 2450 cm−1, respectively, and of phase-phonon bands (at ≈ 1535 and 1300 cm−1 in H. mobilis, at ≈ 1465 and 1280 cm-1 in C. limicola) indicate that the positive charge in the photooxidized primary donor is shared between two coupled BChls. The structure of P840+ in C. limicola, in terms of the resonance interactions between the two BChl a molecules constituting the oxidized primary donor, is close to 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.

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