Pulse ENDOR studies on the radical pair P700+·A1·− and the photoaccumulated quinone acceptor A1·− of photosystem I

• Published in: Applied magnetic resonance 2004-01, Vol.26 (1-2), p.5-21
• Main Authors: Teutloff, C, Bittl, R, Lubitz, W
• Format: Article
• Language: English
• Subjects: Couplings; Density distribution; Electron paramagnetic resonance; Electron transport; Hydrogen bonds; Hydrophobicity; Phylloquinone; Quinones; Superhigh frequencies; Tensors
• ISSN: 0937-9347; 1613-7507
• DOI: 10.1007/BF03166559

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 A1·− and the radical pair state P700·+A1·− 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 A1·− is explained by the hydrophobic binding pocket of this radical. The hyperfine couplings and the spin (and charge) density distribution is very different for A1.− in PS I and VK1·− in frozen alcoholic solution. This is attributed to a rather strong one-sided hydrogen bond to A1·−. The presence of a hydrogen bond to A1·− has only a minor effect ong. The hyperfine coupling constants of A1·− determined from the radical pair spectra deviate only slightly from those derived from photoaccumulated A1·− 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 A1·− was obtained. The significance of the hydrogen bond and other (hydrophobic) interactions of A1 with the surrounding are briefly discussed.