Time-Resolved X-, K-, and W-Band EPR of the Radical Pair State of Photosystem I in Comparison with in Bacterial Reaction Centers

The spin-polarized EPR spectra at 95 GHz (W-band), 24 GHz (K-band), and 9 GHz (X-band) of the radical pair in highly purified photosystem I particles are presented. The spectra are analyzed to obtain both the magnetic parameters of the radical pair as well as the relative orientation of the two species. From the analysis, the g -tensor of is found to be g xx = 2.0062, g yy = 2.0051, and g zz = 2.0022, and it is shown that A 1 is oriented such that the carbonyl bonds are parallel to the vector joining the centers of and . The anisotropy of the g -tensor is considerably larger than that obtained for chemically reduced phylloquinone in frozen 2-propanol solution. Possible reasons for this difference and their implications for the A 1 binding site are discussed. The relative orientation of and is compared with earlier estimates obtained using less accurate g -values for . A comparison with the spectra of in bacterial reaction centers (bRCs) of Rhodobacter sphaeroides R-26 in which the nonheme iron has been replaced by zinc (Zn-bRCs) allows the structural and magnetic properties of the charge-separated state in the two systems to be compared. From the similarity of the two W-band spectra in the region around the free electron g -value it is clear that the dipolar vector, z d, between P •+ and / has a similar orientation relative to P 700 in PS I and P 865 in bRCs. This is compatible with the similar overall structural arrangement of P 700 and P 865 . In contrast, the low-field parts of the two spectra are very different as a result of differences in the orientation of A 1 and Q A with respect to z d .