MATRIX ENDOR OF TYROSINE D+ IN ORIENTED PHOTOSYSTEM-II MEMBRANES

Proton matrix ENDOR spectroscopy has been used to probe the proton environment of the stable tyrosine D+ radical present in Photosystem II membranes. ENDOR spectra in randomly oriented and oriented membranes are reported. Simulations of the spectra are given using both the point dipole approximation and the spin distribution over the tyrosine radical, which were found to provide reasonably good fits. The following results are reported: (1) Five groups of protons can be detected in the matrix ENDOR spectra of D+, that have ENDOR peak separations less than 2 MHz, and are located at distances between 3.5 and 6.7 angstrom from the center of the tyrosine ring. These protons are directed approximately parallel to the PS II membrane plane. (2) Three pairs of ENDOR lines with separations of 2.9, 4.1 and 4.7 MHz, are also reported, together with a signal from the hydrogen bonded to the terminal oxygen in the tyrosine molecule. These protons can be exchanged by treatment in a deuterated buffer. From the orientation dependence of these ENDOR lines, it is concluded that they correspond to protons with distance vectors from the ring center that are perpendicular to the PS II membrane plane. We suggest that this specific organization of protons around the tyrosyl D+ radical contributes to the particular orientation of the beta-CH2 group in the radical, and may also play a role in the unusual stability of the D+ radical form in the PS II membrane.