Light-induced Fourier transform infrared (FTIR) difference spectroscopy has been used to study the photo-oxidation of the primary electron donor (P700) in PS I particles from Chlamydomonas reinhardtii and Synechocystis sp. PCC 6803. To aid in the interpretation of the spectra, PS I particles from a site-directed mutant of C. reinhardtii, in which the axial histidine ligand (HisA676) was changed to serine, were also studied. A high-frequency (3300-2600 cm-1) electronic transition is observed for all PS I particles, demonstrating that P700 is dimeric. The electronic band is, however, species-dependent, indicating some differences in the electronic structure of P700 and/or P700+ in C. reinhardtii and Synechocystis sp. 6803. For PS I particles from C. reinhardtii, substitution of HisA676 with serine has little effect on the ester carbonyl modes of the chlorophylls of P700. However, the keto carbonyl modes are considerably altered. Comparison of (P700+ - P700) FTIR difference spectra obtained using PS I particles from the wild type (WT) and the HS(A676) mutant of C. reinhardtii indicates that the mutation primarily exerts its influence on the P700 ground state. The 13 keto carbonyls of the chlorophylls of P700 of the wild type absorb at similar frequencies, which has previously made these transitions difficult to resolve. However, for the HS(A676) mutant, the 131 keto carbonyl of chlorophyll a or chlorophyll a′ of P700 on PsaB or PsaA absorbs at 1703.4 or 1694.2 cm-1, respectively, allowing their unambiguous resolution. Upon P700+ formation, in both PS I particles from C. reinhardtii, the higher-frequency carbonyl band upshifts by ∼14 cm-1 while the lower frequency carbonyl downshifts by ∼10 cm-1. The similarity in the spectra for WT PS I particles from C. reinhardtii and Synechocystis sp. 6803 indicates that a similar interpretation is probably valid for PS I particles from both species. The mutant results allow for an interpretation of the behavior of the 131 keto carbonyls of P700 that is different from previous work [Breton, J., Nabedryk, E., and Leibl, W. (1999) Biochemistry 38, 11585-11592], in which it was suggested that 131 keto carbonyls of P700 absorb at 1697 and 1639 cm-1, and upshift by 21 cm-1 upon cation formation. The interpretation of the spectra reported here is more in line with recent results from ENDOR spectroscopy and high-resolution crystallography.
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