The Radical Pair State PA in PhotosystemⅠ Single Crystals: Orientation Dependence of the Transient Spin-Polarized EPR Spectra

摘要

The light-induced, charge-separated state P↑(·+)↓(700)A↑(·-)↓(1) in single crystals of PhotosystemⅠ (PSI) from the cyanobacterium Synechococcus elongatus is investigated with transient, direct-detection EPR spectroscopy. The orientation of the phylloquinone head group of A↓(1) 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↑(·-)↓(l) made up of the primary donor, P↓(700), and the acceptor, A↓(l). From the angular dependence of the overall spin-polarization pattern an upper limit ∠(c, Z↓(d)) ≤30° is evaluated for the angle between the crystallographic c-axis, collinear with the membrane normal, and the dipolar axis, Zd, 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↓(‖) principal axis encloses an angle of β= 35°--55° with c. Simulations of the rotation patterns support a lower limit for the angle ∠(c, Zd) ≥ 25° with a larger error than for the upper limit. Zd is confirmed to be parallel to both the g↓(xx) principal axis of g(A↑(·-)↓(1)) and to the C=O carbonyl bonds within ±5° With respect to rotation around the g↓(xx) axis, the angle between the (c, Zd) plane and the quinone plane of the A↑(·-)↓(1) head group can only be specified within an upper limit of 60°. 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.