Quinone exchange at the A site in Photosystem I in spinach and 1 cyanobacteria

Ž . q y Ž . The electron spin polarization ESP EPR signal arising from the P-700 A radical pair in Photosystem I PS I 1 Ž . consisting of the oxidized PS I primary donor, P-700 , and the reduced vitamin K K acceptor, A , is studied as a 1 1 1 function of isotopic labelling of the native A acceptor. No pre-extraction of the native A acceptor is utilized, instead, 1 1 exchange of the native acceptor with protonated or perdeuterated K is accomplished by incubation of the reaction centers 1 Ž . in solution incubationrexchange . The incubationrexchange process is studied in PS I of spinach and in two cyanobacteŽ . Ž ria, the thermophilic Synechococcus liÕidus S. liÕidus and the non-thermophilic Synechococcus leopoliensis S. leopolien. H H H D D H D D Ž sis . A complete data set, P A , P A , P A , P A where P refers to the primary chlorophyll donor, A , the 1 1 1 1 1 . quinone acceptor, and H and D denote protonated and deuterated, respectively , is obtained for S. liÕidus. The correlated Ž . radical pair polarization model CRPP is used to reproduce the experimental lineshapes for the four different isotopic combinations of the P-700A radical pair. The experimental ESP EPR spectra are well predicted using the geometric and 1 Ž energetic parameters which have been deduced for perdeuterated PS I reaction centers in whole cells of S. liÕidus Kothe, Ž . . G. et al. 1994 J. Phys. Chem. 98, 2706–2712 . The results suggest that the isotopically labelled K binds in the A site 1 1 with the same orientation as the native acceptor in unexchanged PS I. No evidence for electron transfer to K in 1 configurations other than the A binding site is observed. Incubationrexchange is studied as a function of incubation 1 temperature. For spinach and non-thermophilic S. leopoliensis the effect of incubationrexchange is most efficient for 1r2 h incubations at temperatures around and above 30–358C, but requires somewhat higher incubation temperatures, above 40–458C, for thermophilic S. liÕidus. Similar changes in the ESP spectrum are also demonstrated for PS I reaction centers in thylakoid membranes of S. liÕidus.