ENDOR and Special TRIPLE Resonance Spectroscopy of QA.bul.- of Photosystem 2

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Abstract
ENDOR and special TRIPLE spectroscopies have been used to study the electron spin density distribution and hydrogen bonding of the plastosemiquinone anion radical, QA.-, of photosystem 2. The semiquinone radical 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) Biochemistry 33, 9922]. H2O/D2O exchange was used to assign hyperfine couplings 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 protein leads to changes in the electron spin density distribution and the hydrogen bond orientation; both hydrogen bonds are parallel to the quinone ring plane in vitro, whereas QA.- has one parallel and one perpendicular to the plane. These results are discussed in the light of previous ENDOR studies of the ubiquinone radical QA.- of Rhodobacter sphaeroides and the predicted structure of the QA-binding region of photosystem 2.

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