Measurement of the midpoint potential of the pheophytin acceptor of photosystem II

Abstract

Primary photochemistry in photosystem II (PSII) involves at least three components: (1) P680, the primary electron donor [ 11, which has been reported to be a chlorophyll dimer [2,3] or trimer [3] although recent evidence favors its identification as a monomeric chlorophyll species [4,5]; (2) Q, a special plastoquinone [6], which functions as a relatively stable acceptor (200-600 ps at room temperature when secondary electron transport is not blocked [7]); (3) Ph, a pheophytin molecule which functions as an intermediate electron carrier between P680 and Q. The involvement of pheophytin as an electron acceptor in PSI1 was initially demonstrated in [8-lo]. Reversible photo-reduction of pheophytin in PSI1 was observed when Q was chemically reduced. At the same time an unsplit EPR radical signal attributed to Phwas reported [lo]. Later it was shown that a split EPR signal could be photo-induced under some circumstances and this was attributed to the reduced Ph which was involved in an interaction with the semiquinone-iron form of Q [ 1 l] by analogy to the situation in photosynthetic bacteria [ 121. Similar EPR [S] and optical spectra at 200 K [ 131 were observed in a different PSI1 reaction center preparation. These spectra were also attributed to a pheophytin anion. Recent ENDOR measurements are consistent with the identification of the reduced intermediate acceptor in PSI1 as a pheophytin anion [ 141. Also a picosecond time-resolved optical spectrum of Phhas been reported in PSI1 particles in PSI1 particles in which Q is chemically reduced [ 1 s]. Strong evidence for the presence of an intermediate acceptor between P680 and Q has also come from the discovery of a reaction center triplet state in PSI1