ENDOR and special triple resonance spectroscopy of A(1)(center dot-) of photosystem

The photoaccumulated radical state of the photosystem 1 secondary electron acceptor A(1), A(1)(.-), has been studied in spinach and the cyanobacterium Anabaena variabilis strain Met27 using electron nuclear double resonance (ENDOR) and electron-nuclear-nuclear special triple (ST) resonance spectroscopies. Spectra of A(1)(.-) in both these species are very similar. ENDOR spectra of the phylloquinone anion radical in solvent glass were also obtained. Comparison of the spectra of the in vivo and in vitro radicals shows that A(1)(.-) is a phylloquinone anion radical with a distorted electron spin density distribution. Hyperfine couplings to the A(1)(.-) methyl group and to two protons hydrogen bonded to the quinone oxygens have been identified using biosynthetic deuteration in A. variabilis. Possible hyperfine coupling to a methylene proton of the phytyl side chain of the quinone has also been identified. These results are compared with those from previous studies of protein-bound semiquinones in the light of the unusual redox potential of A(1).