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.
|Number of pages||12|
|Journal||Journal of Physical Chemistry B|
|Publication status||Published - Oct 15 1998|
ASJC Scopus subject areas
- Physical and Theoretical Chemistry