MICROENVIRONMENTS OF TYROSINE D-II STUDIED BY PROTON MATRIX ENDOR( AND TYROSINE Z(+) IN PHOTOSYSTEM)

Proton matrix ENDOR spectra of tyrosines Z(+)(Y-Z(+)) and D+(Y-D(+)) w ere studied in the frozen Tris-treated PS II. ENDOR spectra of the tyr osines in oriented membranes observed at 10 K have shown similar angul ar dependence with similar line pair separations, suggesting that the local arrangement of protons surrounding both radical species is simil ar. The angular dependence of EPR spectra showed the C-O bonds in Y-Z( +) and Y-D(+) lie parallel to the membrane normal. With increasing tem perature, the intensity of the line pair with 1.2 MHz separation decre ased above 25 K for Y-Z(+), and above 110 K for Y-D(+), respectively. Assuming a motion of the proton responsible to the line pair, the acti vation energies for the motional broadening were estimated to be 86 +/ - 20 J/mol for Y-Z(+), and 810 +/- 200 J/mol for Y-D(+), respectively. The broadening may be ascribed to small amplitude vibrations of proto ns surrounding tyrosine radicals. The intensities for the peaks with a bout 7 MHz separation assigned to the OH proton of the tyrosine decrea sed, which was ascribed to a similar motional broadening. The differen ce in temperature dependence of line broadening of the two radicals su ggests that the functional difference between them may be ascribable t o differences in flexibility in their microenvironment.