The dark stable tyrosine radical of photosystem 2 studied in three species using ENDOR and EPR spectroscopies.

The dark stable neutral tyrosine radical YD. of photosystem 2 (PS2) has been studied using electron nuclear double-resonance (ENDOR) and electron paramagnetic resonance (EPR) spectroscopies. The proton hyperfine coupling constants of all four ring protons and both beta-methylene protons have been determined for YD. in three species covering the range of oxygenic organisms; a higher plant (spinach), an alga (Chlamydomonas reinhardtii), and a cyanobacterium (Phormidium laminosum). It has generally been assumed that the properties of Yd. 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 species dependent while the ring proton coupling constants are not. Estimation of the electron spin density distribution of Yd. from all three organisms has been done. This shows that changes in beta-proton coupling constants in each organism arise from the slightly different orientation of the tyrosine ring, relative to the beta-protons. The electron spin density distribution within the tyrosine ring is organism independent. The variations in the beta-proton coupling constants are reflected in the corresponding EPR spectra, where small variations in line width have been detected. These data delineate the range of natural variation in the spectroscopic properties of YD., and by assigning the features of the ENDOR spectrum, provide a basis for both the unification of studies of YD. in different organisms and the study of YZ.. The results are discussed in relation to data in the recent study (Hoganson & Babcock, 1992) using YD. in the cyanobacterium, Synechocystis PCC 6803.