Low-temperature resonance Raman spectra of D1/D2 particles from spinach excited at 406 and 413 nm contained enhanced contributions from pheophytin a. These contributions were partly photobleachable in dithionite-treated particles. Difference resonance Raman spectra calculated on this basis essentially arose from a single environmental population of neutral pheophytin a. The 9-keto carbonyls of these bleachable molecules vibrated at 1680 cm−1, a frequency identical, within experimental uncertainty, with that of the 9-keto carbonyl of the acceptor bacteriopheophytin HL in the reaction centers of Rhodopseudomonas viridis and Rhodobacter sphaeroides. This constitutes strong evidence that the acceptor pheophytin of the PS II reaction center is H-bonded to the D1-130 glutamic residue, in a relative geometry and environment that must be very close to those of the HL molecule and the L-104 glutamic residue in the reaction centers of the above two bacterial species.
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