ENDOR AND SPECIAL TRIPLE-RESONANCE SPECTROSCOPY OF QA(CENTER-DOT-) OFPHOTOSYSTEM-2

ENDOR and special TRIPLE spectroscopies have been used to study the el ectron spin density distribution and hydrogen bonding of the plastosem iquinone anion radical, Q(A)(.-) Of photosystem 2. The semiquinone rad ical was made accessible to ENDOR through the use of exogenous cyanide , which decouples the radical from the ferrous iron of the photosystem 2 ferroquinone acceptor complex [Sanakis, Y., et al. (1994) Biochemis try 33, 9922]. H2O/D-2?O exchange was used to assign hyperfine couplin gs to hydrogen-bonded protons, and orientation-selected special TRIPLE spectroscopy has revealed the orientation of hydrogen bonds relative to the quinone ring. Methyl group resonances have also been assigned. ENDOR spectra of the decylplastosemiquinone anion radical in vitro are presented for comparison. This shows that interaction with the protei n leads to changes in the electron spin density distribution and the h ydrogen bond orientation; both hydrogen bonds are parallel to the quin one ring plane in vitro, whereas Q(A)(.-) has one parallel and one per pendicular to the plane. These results are discussed in the light of p revious ENDOR studies of the ubiquinone radical Q(A)(.-) Of Rhodobacte r sphaeroides and the predicted structure of the QA-binding region of photosystem 2.