title: FT-IR studies on the triplet state of P680 in the photosystem II reaction center: triplet equilibrium within a chlorophyll dimer.
abstract: 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 (3P680). The 3P680/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 3P680/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.
authors.affiliation: Solar Energy Research Group, Institute of Physical and Chemical Research (RIKEN), Saitama, Japan.
authors.forename: T
authors.initials: T
authors.lastname: Noguchi
authors.forename: Y
authors.initials: Y
authors.lastname: Inoue
authors.forename: K
authors.initials: K
authors.lastname: Satoh
mesh_terms: D002734:Chlorophyll / Q000737:chemistry*; D005583:Fourier Analysis; D006860:Hydrogen Bonding; D045342:Light-Harvesting Protein Complexes; D045322:Photosynthetic Reaction Center Complex Proteins / Q000737:chemistry*; D045332:Photosystem II Protein Complex; D011487:Protein Conformation; D013055:Spectrophotometry, Infrared; D013696:Temperature
chemical_list: D045342:Light-Harvesting Protein Complexes; D045322:Photosynthetic Reaction Center Complex Proteins; D045332:Photosystem II Protein Complex; D002734:Chlorophyll