The radical pair state P ^.+ ₇₀₀A ^.- ₁ in photosystem I single crystals: Orientation dependence of the transient spin-polarized EPR spectra

The light-induced, charge-separated state P ^.+ ₇₀₀A ^.- 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 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 ^.+ ₇₀₀A ^.- 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 〈(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 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 _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(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, z _d) 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.