FT-IR STUDIES ON THE TRIPLET-STATE OF P680 IN THE PHOTOSYSTEM-II REACTION-CENTER - TRIPLET EQUILIBRIUM WITHIN A CHLOROPHYLL DIMER

The structure and molecular interactions of the primary donor (P680) in the reaction center (D1-D2-cytochrome b-559 complex) of photosystem II (PS II) have been investigated by detecting light-induced FT-IR difference spectra upon the formation of its triplet state (P-3(680)). The P-3(680)/P680 spectrum obtained was analyzed by comparing it with difference spectra between the ground and lowest triplet states of purified chlorophyll a (Chl) in organic solvents. The negative peaks at 1669 and 1707 cm-1 accompanied by the positive peaks at 1627 and 1659 cm-1 in the P-3(680)/P680 spectrum were assigned to the keto C=O stretching mode, and the appearance of these two pairs of bands indicated that P680 has a dimeric structure analogous to that of the bacterial primary donor. From the band positions of the keto and carbomethoxy C=O stretches, the hydrogen-bonding properties of these two Chl molecules were found to be asymmetrical; in one Chl molecule both the keto and carbomethoxy C=O groups form hydrogen bonds, while in the other Chl molecule the keto C=O is not hydrogen-bonded whereas the carbomethoxy C=O probably is hydrogen-bonded. The temperature dependence of the intensity ratios of the keto C=O bands revealed that the triplet state is equilibrated between the two Chl molecules with an energy gap of 8.4 +/- 0.7 meV. Most of the triplet population was found to be localized on one Chl molecule (86% at 80 K), in which both of the two C=O groups are hydrogen-bonded, that is probably attached to the D1 subunit. Considering the structure of the bacterial reaction center determined by X-ray crystallography and the sequence homology between the D1 and D2 subunits of PS II and the L and M subunits of bacteria, a model of the P680 structure and its interactions with apoproteins has been proposed.