Pulse ENDOR studies on the radical pair P 700 +· A 1 ·− and the photoaccumulated quinone acceptor A 1 ·− of photosystem I

Abstract

The secondary acceptor A1 of the electron transport chain(s) of photosystem (PS) I is a phylloquinone (vitamin K1, VK1). Pulse electron paramagnetic resonance and electron nuclear double resonance (ENDOR) experiments at X-band frequencies were performed on the photoaccumulated acceptor radical A 1 ·− and the radical pair state P 700 ·+ A 1 ·− in PS I ofThermosynechococcus elongatus. The data obtained were compared with data from the respective radical anion of VK1 in organic solvents. The unusualg tensor magnitude of A 1 ·− is explained by the hydrophobic binding pocket of this radical. The hyperfine couplings and the spin (and charge) density distribution is very different for A 1 .− in PS I and VK 1 ·− in frozen alcoholic solution. This is attributed to a rather strong one-sided hydrogen bond to A 1 ·− . The presence of a hydrogen bond to A 1 ·− has only a minor effect ong. The hyperfine coupling constants of A 1 ·− determined from the radical pair spectra deviate only slightly from those derived from photoaccumulated A 1 ·− in PS I treated with dithionite at high pH. ENDOR resonances of the proton in a H bond were detected and an estimate of the strength and geometry of this bond to A 1 ·− was obtained. The significance of the hydrogen bond and other (hydrophobic) interactions of A1 with the surrounding are briefly discussed.