ENDOR AND SPECIAL TRIPLE-RESONANCE SPECTROSCOPY OF A(1)(CENTER-DOT-) OF PHOTOSYSTEM

The photoaccumulated radical state of the photosystem 1 secondary elec tron acceptor A(1), A(1)(.-), has been studied in spinach and the cyan obacterium Anabaena variabilis strain Met27 using electron nuclear dou ble resonance (ENDOR) and electron-nuclear-nuclear special triple (ST) resonance spectroscopies. Spectra of A(1)(.-) in both these species a re very similar. ENDOR spectra of the phylloquinone anion radical in s olvent glass were also obtained. Comparison of the spectra of the in v ivo and in vitro radicals shows that A(1)(.-) is a phylloquinone anion radical with a distorted electron spin density distribution. Hyperfin e couplings to the A(1)(.-) methyl group and to two protons hydrogen b onded to the quinone oxygens have been identified using biosynthetic d euteration in A. variabilis. Possible hyperfine coupling to a methylen e proton of the phytyl side chain of the quinone has also been identif ied. These results are compared with those from previous studies of pr otein-bound semiquinones in the light of the unusual redox potential o f A(1).