Pulsed EPR studies of doublet signal and singlet-like signal in oriented Ca2+-depleted PSII membranes: Location of the doublet signal center in PSII

Doublet signal and singlet-like signal induced in Ca2+-depleted PS II were investigated by pulsed EPR in one-dimensionally oriented photosystem (PS) I I membranes. The doublet signal showed marked angular dependent change in i ts spectrum in term of the applied magnetic field, indicating that the magn etic dipole-dipole interaction is mainly responsible for the doublet signal . The singlet-like signal also showed angular dependence, which was less pr onounced than that of the doublet signal. The parameters of dipole and exch ange interactions used to simulate the doublet signal indicate that the sig nal arises from a magnetically coupled organic radical pair. Angular depend ence of the doublet signal indicates that the radius vector of the radical pair (r) and the normal of the thylakoid membrane is at an angle of 65 degr ees. Pulsed ELDOR studies in the oriented membranes indicate that the vecto r (R) connecting the doublet-signal center with the Y-D(.) radical and the plane of the thylakoid membrane are at an angle of 8 degrees, Furthermore, the angle between the projections of the R and r vectors on the plane of th e thylakoid membrane was determined to be 64 degrees. The location of the d oublet-signal species in PS II is discussed.