A CHLOROPHYLL TILTED 30-DEGREES RELATIVE TO THE MEMBRANE IN THE PHOTOSYSTEM-II REACTION CENTER

The orientation properties of the reaction centre triplet in Photosystem II (PS II) were determined. A high triplet yield was generated in PS II membranes by double reduction of the primary quinone electron acceptor Q(A). It is deduced that the triplet state is localised on a chlorophyll, the tetrapyrrolic plane of which is tilted at 30-degrees to the membrane. A similar orientation was found in D1/D2/cyt b-559 particles demonstrating that the triplet is confined to the reaction centre. Optical work in the literature has been interpreted as indicating that the triplet is localised on a monomeric chlorophyll and that, in the singlet state, P680 consists of this molecule weakly coupled to a second chlorophyll. The weakness of the coupling, compared to the coupling in the special pair of purple bacteria, allows P680 to be considered as a monomer. Taking the optical data into account, we propose that P680 is a chlorophyll molecule oriented at 30-degrees to the membrane. This result is discussed in terms of the structural analogy between PS II and the reaction centre of purple bacteria. A model is favored in which P680 is a chlorophyll, structurally analogous to one of the monomeric bacteriochlorophylls of the bacterial reaction centre. In addition, the orientation data indicate that this chlorophyll is rotated by 45-degrees in its ring plane compared to the monomeric bacteriochlorophylls in the reaction centre of Rhodopseudomonas viridis.