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-protein. Upon oxidation it forms an unusually stable neutral radical with characteristic EPR properties. We prepared highly active BBY particles 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 spinach and 19.0 G for Scenedesmus. Power saturation experiments were performed at 20 K. For unknown reasons, the signal of spinach satures slightly 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 membrane normal is similar in both species.