Site-directed mutations were introduced to replace D1-His198 and D2-His197 of the D1 and D2 polypeptides, respectively, of the photosystem II (PSII) reaction center of Synechocystis PCC 6803. These residues coordinate chlorophylls PA and PB which are homologous to the special pair Bchlorophylls of the bacterial reaction centers that are coordinated respectively by histidines L-173 and M-200 (202). PA and PB together serve as the primary electron donor, P, in purple bacterial reaction centers. In PS II, the site-directed mutations at D1 His198 affect the P+-P-absorbance difference spectrum. The bleaching maximum in the Soret region (in WT at 433 nm) is blue-shifted by as much as 3 nm. In the D1 His198Gln mutant, a similar displacement to the blue is observed for the bleaching maximum in the Qy region (672.5 nm in WT at 80 K), whereas features attributed to a band shift centered at 681 nm are not altered. In the Yz•-Yz-difference spectrum, the band shift of a reaction center chlorophyll centered in WT at 433-434 nm is shifted by 2-3 nm to the blue in the D1-His198Gn mutant. The D1-His198Gln mutation has little effect on the optical difference spectrum, 3P-1P, of the reaction center triplet formed by P+Pheo- charge recombination (bleaching at 681-684 nm), measured at 5-80 K, but becomes visible as a pronounced shoulder at 669 nm at temperatures ≥ 150 K. Measurements of the kinetics of oxidized donor-QA- charge recombination and of the reduction of P+ by redox active tyrosine, Yz, indicate that the reduction potential of the redox couple P+/P can be appreciably modulated both positively and negatively by ligand replacement at D1-198 but somewhat less so at D2-197. On the basis of these observations and others in the literature, we propose that the monomeric accessory chlorophyll, BA, is a long-wavelength trap located at 684 nm at 5 K. BA * initiates primary charge separation at low temperature, a function that is increasingly shared with PA * in an activated process as the temperature rises. Charge separation from BA * would be potentially very fast and form PA+BA- and/or BA+Pheo- as observed in bacterial reaction centers upon direct excitation of BA (van Brederode, M. E., et al. (1999) Proc. Natl. Acad Sci. 96, 2054-2059). The cation, generated upon primary charge separation in PSII, is stabilized at all temperatures primarily on PA, the absorbance spectrum of which is displaced to the blue by the mutations. In WT, the cation is proposed to be shared to a minor extent (∼20%) with PB, the contribution of which can be modulated up or down by mutation. The band shift at 681 nm, observed in the P+-P difference spectrum, is attributed to an electrochromic effect of PA+ on neighboring BA. Because of its low-energy singlet and therefore triplet state, the reaction center triplet state is stabilized on BA at ≤80 K but can be shared with PA at >80 K in a thermally activated process.
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