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

Abstract

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 A1 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, P700, and the acceptor, A1. From the angular dependence of the overall spin-polarization pattern an upper limit ∠(c, zd) ≤ 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 and . A partially resolved hyperfine coupling (hfc) is assigned to the hfc tensor of the 2-methyl group of A1. 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 gxx principal axis of g( ) and to the CO carbonyl bonds within ±5°. With respect to rotation around the gxx axis, the angle between the (c, zd) 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 A1 within the PSI reaction center, a nearly complete structural model for the head group of the functional A1 cofactor is achieved.