THE DARK STABLE TYROSINE RADICAL OF PHOTOSYSTEM-2 STUDIED IN 3 SPECIES USING ENDOR AND EPR SPECTROSCOPIES

The dark stable neutral tyrosine radical Y-D. of photosystem 2 (PS2) h as been studied using electron nuclear double-resonance (ENDOR) and el ectron paramagnetic resonance (EPR) spectroscopies. The proton hyperfi ne coupling constants of all four ring protons and both beta-methylene protons have been determined for Y-D. in three species covering the r ange of oxygenic organisms; a higher plant (spinach), an alga (Chlamyd omonas reinhardtii), and a cyanobacterium (Phormidium laminosum). It h as generally been assumed that the properties of Y-D. are the same in all oxygenic organisms, while in fact there are small but significant differences. The beta-proton coupling constants are shown to be specie s dependent while the ring proton coupling constants are not. Estimati on of the electron spin density distribution of Y-D. from all three or ganisms has been done. This shows that changes in beta-proton coupling constants in each organism arise from the slightly different orientat ion of the tyrosine ring, relative to the beta-protons. The electron s pin density distribution within the tyrosine ring is organism independ ent. The variations in the beta-proton coupling constants are reflecte d in the corresponding EPR spectra, where small variations in line wid th have been detected. These data delineate the range of natural varia tion in the spectroscopic properties of Y-D., and by assigning the fea tures of the ENDOR spectrum, provide a basis for both the unification of studies of Y-D. in different organisms and the study of Y-Z.. The r esults are discussed in relation to data in the recent study (Hoganson & Babcock, 1992) using Y-D. in the cyanobacterium, Synechocystis PCC 6803.