TIME-RESOLVED X-BAND, K-BAND, AND W-BAND EPR OF THE RADICAL PAIR STATE P(700)(CENTER-DOT-DOT-) OF PHOTOSYSTEM-I IN COMPARISON WITH P(865)(CENTER-DOT+)Q(A)(CENTER-DOT-) IN BACTERIAL REACTION CENTERS()A(1)(CENTER)

The spin-polarized EPR spectra at 95 GHz (W-band), 24 GHz (K-band), an d 9 GHz (X-band) of the radical pair P(700)(.+)A(1)(.-) in highly puri fied photosystem I particles are presented. The spectra are analyzed t o obtain both the magnetic parameters of the radical pair as well as t he relative orientation of the two species. From the analysis, the g-t ensor of A(1)(.-) is found to be g(xx) = 2.0062, g(yy) = 2.0051, and g (zz) = 2.0022, and it is shown that Al is oriented such that the carbo nyl bonds are parallel to the vector joining the centers of P-700(.+) and A, The anisotropy of the g-tensor is considerably larger than that obtained for chemically reduced phylloquinone in frozen 2-propanol so lution. Possible reasons for this difference and their implications fo r the Al binding site are discussed. The relative orientation of P-700 (.+), and A(1)(.-) is compared with earlier estimates obtained using l ess accurate g-values for A(1)(.-). A comparison with the spectra of P (865)(.+)Q(A)(.-) in bacterial reaction centers (bRCs) of Rhodobacter sphaeroides R-26 in which the nonheme iron has been replaced by zinc ( Zn-bRCs) allows the structural and magnetic properties of the charge-s eparated state in the two systems to be compared.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-val ue it is clear that the dipolar vector, z(d), between P-.+ and Q(A)(.- )/A(1)(.-) has a similar orientation relative to P-700 in PSI and P-86 5 in bRCs. This is compatible with the similar overall structural arra ngement 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 orientat ion of Al and QA With respect to z(d).