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 h igh-field EPR spectroscopy. Car(.+) was generated by 5 K illumination in th e magnet. The spectrum was characterized by a g-tensor, the principal value s 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 consist ent with the electron transfer from Chl(Z) to Car(.+) that occurs upon rais ing the temperature as proposed earlier [Hanley et al., Biochemistry, 1999, 38, 8189]. The spectra of the one-dimensional oriented samples were obtain ed 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-c haracterized bacteriochlorophyll a, the data show that the ring plane of Ch l(Z)(.+) is oriented perpendicular to the membrane plane.