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FTIR study of the primary electron donor of photosystem I (P700) revealing delocalization of the charge in P700(+) and localization of the triplet character in (3)P700

FTIR study of the primary electron donor of photosystem I (P700) revealing delocalization of the charge in P700(+) and localization of the triplet character in (3)P700

Biochemistry 38(36): 11585-11592

The effect of global 15N or 2H labeling on the light-induced P700+/P700 FTIR difference spectra has been investigated in photosystem I samples from Synechocystis at 90 K. The small isotope-induced frequency shifts of the carbonyl modes observed in the P700+/P700 spectra are compared to those of isolated chlorophyll a. This comparison shows that bands at 1749 and 1733 cm-1 and at 1697 and 1637 cm-1, which upshift upon formation of P700+, are candidates for the 10a-ester and 9-keto C[double bond]O groups of P700, respectively. A broad and relatively weak band peaking at 3300 cm-1, which does not shift upon global labeling or 1H-2H exchange, is ascribed to an electronic transition of P700+, indicating that at least two chlorophyll a molecules (denoted P1 and P2) participate in P700+. Comparisons of the 3P700/P700 FTIR difference spectrum at 90 K with spectra of triplet formation in isolated chlorophyll a or in RCs from photosystem II or purple bacteria identify the bands at 1733 and 1637 cm-1, which downshift upon formation of 3P700, as the 10a-ester and 9-keto C[double bond]O modes, respectively, of the half of P700 that bears the triplet (P1). Thus, while the P2 carbonyls are free from interaction, both the 10a-ester and the 9-keto C[double bond]O of P1 are hydrogen bonded and the latter group is drastically perturbed compared to chlorophyll a in solution. The Mg atoms of P1 and P2 appear to be five-coordinated. No localization of the triplet on the P2 half of P700 is observed in the temperature range of 90-200 K. Upon P700 photooxidation, the 9-keto C[double bond]O bands of P1 and P2 upshift by almost the same amount, giving rise to the 1656(+)/1637(-) and 1717(+)/1697(-) cm-1 differential signals, respectively. The relative amplitudes of these differential signals, as well as of those of the 10a-ester C[double bond]O modes, appear to be slightly dependent on sample orientation and temperature and on the organism used to generate the P700+/P700 spectrum. If it is assumed that the charge density on ring V of chlorophyll a, as measured by the perturbation of the 10a-ester or 9-keto C[double bond]O IR vibrations, mainly reflects the spin density on the two halves of the oxidized P700 special pair, a charge distribution ranging from 1:1 to 2:1 (in favor of P2) is deduced from the measurements presented here. The extreme downshift of the 9-keto C[double bond]O group of P1, indicative of an unusually strong hydrogen bond, is discussed in relation with the models previously proposed for the PSI special pair. Copyright 1999, American Chemical Society.

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Accession: 009847308

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PMID: 10512612

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