RELATIVE ORIENTATION OF THE OPTICAL-TRANSITION DIPOLE AND TRIPLET AXES OF THE PHOTOSYSTEM-II PRIMARY DONOR - A MAGNETOPHOTOSELECTION STUDY

A magnetophotoselection (MPS) study on the triplet state of the primary donor in the D1-D2-cytochrome b559 complex is performed at 6 K. For this purpose a novel double-modulation technique was developed, resulting in a 2-fold better signal-to-noise ratio, compared to the conventional technique. The MPS data were found to depend on the excitation wavelength, The angles calculated from the MPS at the longest wavelength are representative for the a,transition moment of P680, phi(x) = 55 degrees +/- 1 degrees, phi(y) = 30 degrees +/- 4 degrees, phi(z) = 90 degrees +/- 12 degrees, where phi(i) is the angle with the ith triplet axis, and yield a positive sign for E. The angles found for P680 differ from those reported for monomeric chlorophyll a in solution (Vrieze, J.; Hoff, A. J. Chem. Phys. Lett. 1995, 237, 493). This difference is discussed in terms of the structure of P680. Assuming C-2 symmetry, with the 2-fold axis perpendicular to the membrane plane, a model is presented in which P680 is an excitonically-coupled dimer of chlorophyll a molecules, whose triplet state at 6 K resides on one of the constituent monomers. The angles of the triplet magnetic axes of this monomer with the membrane plane correspond to those determined in an EPR study on oriented samples (Van Mieghem, F. J. E.; Satoh, K.; Rutherford, A, W. Biochim. Biophys. Acta 1991, 1058, 379-385). The ($) over right arrow Q(y) transition moments of the two chlorophylls of P680 are close to antiparallel, and the ($) over right arrow Q(y)((-)) axis of the lower excitonic component lies in the membrane plane. The Chi porphyrin planes make an angle of 60 degrees, and their intercenter distance is estimated to be 11 Angstrom.