A resonance raman characterization of the primary electron acceptor in photosystem II

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 H_L 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 H_L molecule and the L-104 glutamic residue in the reaction centers of the above two bacterial species.