High-Field EPR Study of Carotenoid•+ and the Angular Orientation of Chlorophyll z•+ in Photosystem II

The carotenoid (Car •+ ) and chlorophyll z cation (Chl Z •+ ) radicals in hydroxylamine treated PSII membranes from spinach were studied by 285 GHz high-field EPR spectroscopy. Car •+ was generated by 5 K illumination in the magnet. The spectrum was characterized by a g-tensor, the principal values of which were 2.00322, 2.00252, and 2.00211. The spectrum of the Chl Z •+ generated by illumination at 198 K was clearly different from the Car •+, showing less resolution. The g-values were 2.00308, 2.00253, and 2.00216. An identical Chl Z •+ spectrum was obtained when the sample exhibiting the Car •+ spectrum was dark-adapted at 198 K. This observation is consistent with the electron transfer from Chl Z to Car •+ that occurs upon raising the temperature as proposed earlier [Hanley et al., Biochemistry, 1999, 38, 8189]. The spectra of the one-dimensional oriented samples were obtained to determine the angular orientation of Chl Z •+ . If one assumes that the g-tensor of the Chl Z cation radical is oriented similar to the well-characterized bacteriochlorophyll a, the data show that the ring plane of Chl Z •+ is oriented perpendicular to the membrane plane.