Rigby S E, Heathcote P, Evans M C, Nugent J H
Department of Biology, University College London, UK.
Biochemistry. 1995 Sep 19;34(37):12075-81. doi: 10.1021/bi00037a051.
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.
电子核双共振（ENDOR）和特殊的三重光谱技术已被用于研究光系统2的质体半醌阴离子自由基QA.-的电子自旋密度分布和氢键。通过使用外源性氰化物，使半醌自由基能够进行ENDOR研究，氰化物可使该自由基与光系统2铁醌受体复合物中的亚铁离子解耦[萨纳基斯，Y.等人（1994年）《生物化学》33卷，9922页]。利用H2O/D2O交换来确定与氢键合质子的超精细偶合，并且取向选择特殊三重光谱揭示了氢键相对于醌环的取向。甲基基团共振也已被确定。给出了体外癸基质体半醌阴离子自由基的ENDOR光谱以供比较。这表明与蛋白质的相互作用导致电子自旋密度分布和氢键取向发生变化；在体外，两个氢键都与醌环平面平行，而QA.-有一个与平面平行，一个与平面垂直。根据之前对球形红细菌的泛醌自由基QA.-的ENDOR研究以及光系统2的QA结合区域的预测结构对这些结果进行了讨论。
