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

The light-induced, charge-separated state 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 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 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 ∠( c, z d ) ≤ 30° 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 and . 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, z d ) ≥ 25° 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 ( ) and to the C O carbonyl bonds within ±5°. With respect to rotation around the g xx axis, the angle between the ( c, z d ) plane and the quinone plane of the 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.