COMPARISON OF ELECTRON-PARAMAGNETIC-RESONANCE LINESHAPE, ORIENTATION AND POWER SATURATION OF THE TYR(D) RADICAL FROM SPINACH AND THE GREEN-ALGA SCENEDESMUS-OBLIQUUS

Tyr(D) is a redox-active amino acid of the Photosystem II (PS II) D2-p rotein. Upon oxidation it forms an unusually stable neutral radical wi th characteristic EPR properties. We prepared highly active BBY partic les of spinach and the green alga Scenedesmus obliquus. The X-band EPR spectra are similar with a peak to trough linewidth of 19.3 G for spi nach and 19.0 G for Scenedesmus. Power saturation experiments were per formed at 20 K. For unknown reasons, the signal of spinach satures sli ghtly faster than that of the alga. Partially oriented PS II membrane particles exhibit comparable orientation-dependent four-line spectra. This indicates that the orientation of Tyr(D) with respect to the memb rane normal is similar in both species.