The radical pair state P700•+A1•- in photosystem I single crystals:: Orientation dependence of the transient spin-polarized EPR spectra

The light-induced, charge-separated state P(700)(.+)A(1)(.-) in single crystals of Photosystem I (PSI) from the cyanobacterium Synechococcus elongatus is investigated with transient, direct-detection EPR spectroscopy, The orientation of the phylloquinone head group of Al within the PSI reaction center is determined from the orientation dependence of the spin-polarized X-band EPR spectrum of the radical pair P(700)(.+)A(1)(.-) made up of the primary donor, P-700, and the acceptor, A(1). From the angular dependence of the overall spin-polarization pattern an upper limit angle(c, z(d)) less than or equal to 30 degrees is evaluated for the angle between the crystallographic c-axis, collinear with the membrane normal, and the dipolar axis, z(d), connecting the electron spin density centers of P-700(.+) and A(1)(.-). A partially resolved hyperfine coupling (hfc) is assigned to the hfc tensor of the 2-methyl group of A(1). Its A(parallel to) principal axis encloses an angle of beta = 35 degrees-55 degrees with c. Simulations of the rotation patterns support a lower limit for the angle angle(c, z(d)) greater than or equal to 25 degrees with a larger error than for the upper limit. z(d) is confirmed to be parallel to both the g(xx) principal axis of g(A(1)(.-)) and to the C=O carbonyl bonds within 15 degrees. With respect to rotation around the g(xx) axis, the angle between the (c, z(d)) plane and the quinone plane of the A(1)(.-) head group can only be specified within an upper limit of 60 degrees. Together with independent knowledge about the location of A(1) within the PSI reaction center, a nearly complete structural model for the head group of the functional A(1) cofactor is achieved.