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 primar y donor in the D1-D2-cytochrome b559 complex is performed at 6 K. For this purpose a novel double-modulation technique was developed, result ing in a 2-fold better signal-to-noise ratio, compared to the conventi onal technique. The MPS data were found to depend on the excitation wa velength, The angles calculated from the MPS at the longest wavelength are representative for the a,transition moment of P680, phi(x) = 55 d egrees +/- 1 degrees, phi(y) = 30 degrees +/- 4 degrees, phi(z) = 90 d egrees +/- 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 diffe r 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, w ith the 2-fold axis perpendicular to the membrane plane, a model is pr esented in which P680 is an excitonically-coupled dimer of chlorophyll a molecules, whose triplet state at 6 K resides on one of the constit uent monomers. The angles of the triplet magnetic axes of this monomer with the membrane plane correspond to those determined in an EPR stud y on oriented samples (Van Mieghem, F. J. E.; Satoh, K.; Rutherford, A , W. Biochim. Biophys. Acta 1991, 1058, 379-385). The ($) over right a rrow Q(y) transition moments of the two chlorophylls of P680 are close to antiparallel, and the ($) over right arrow Q(y)((-)) axis of the l ower 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.