THE RADICAL PAIR STATE P(700)(CENTER-DOT-DOT-) IN PHOTOSYSTEM-I SINGLE-CRYSTALS - ORIENTATION DEPENDENCE OF THE TRANSIENT SPIN-POLARIZED EPR-SPECTRA()A(1)(CENTER)

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 spectr oscopy, The orientation of the phylloquinone head group of Al within t he PSI reaction center is determined from the orientation dependence o f 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 eval uated for the angle between the crystallographic c-axis, collinear wit h the membrane normal, and the dipolar axis, z(d), connecting the elec tron spin density centers of P-700(.+) and A(1)(.-). A partially resol ved hyperfine coupling (hfc) is assigned to the hfc tensor of the 2-me thyl group of A(1). Its A(parallel to) principal axis encloses an angl e 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 th an 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 res pect 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 s pecified within an upper limit of 60 degrees. Together with independen t knowledge about the location of A(1) within the PSI reaction center, a nearly complete structural model for the head group of the function al A(1) cofactor is achieved.