Magnetic-Field-Induced Orientation of Photosynthetic Reaction Centers, As Revealed by Time-Resolved D-Band Electron Paramagnetic Resonance of Spin-Correlated Radical Pairs. II. Field Dependence of the Alignment

The magnetic-field-induced orientation of photosynthetic reaction centers has been studied by time-resolved D-band (130 GHz) electron paramagnetic resonance (EPR) of the secondary radical pair, , in plant photosystem I (PSI). Experiments have been performed for fresh and lyophilized whole cells of the deuterated cyanobacterium S. lividus. A computer fit of the angle-dependent D-band spectra, measured for two different sample orientations, provides the order parameter S Z ‘ Z ‘ of the symmetry axis, Z ‘, of the susceptibility tensor, relative to the magnetic field. The positive sign of this order parameter indicates that membrane proteins are the major source for the anisotropy of the diamagnetic susceptibility, ΔχV. A value for ΔχV has been extracted from the magnetic-field dependence of S Z ‘ Z ‘. The value of ΔχV = 5.7 × 10-27 m3 is in good agreement with an estimate for the susceptibility anisotropy of a cyanobacterial cell. This demonstrates that whole cells are aligned in the magneto-orientation process. The combination of high-field EPR of a magnetically oriented sample with the analysis of quantum beat oscillations allows determination of the three-dimensional structure of in the photosynthetic membrane.