Malkin R, Bearden A J
Biochim Biophys Acta. 1975 Aug 11;396(2):250-9. doi: 10.1016/0005-2728(75)90039-0.
Electron paramagnetic resonance studies of the primary reactants of Photosystems I and II have been conducted at cryogenic temperatures after laser-flash activation with monochromatic light.P-700 photooxidation occurs irreversibly in chloroplasts and in Photosystem I fragments after activation with a 730 nm laser flash at a temperature of 35 degrees K. Flash activation of chloroplasts or Photosystem II chloroplast fragments with 660 nm light results in the production of a free-radical signal (g = 2.002, linewidth approximately 8 gauss) which decays with a half-time of 5.0 ms at 35 degrees K. The half-time of decay is independent of temperature in the range of 10-77 degrees K. This reversible signal can be eliminated by preillumination of the sample at 35 degrees K with 660 nm light (but not by 730 nm light), by preillumination with 660 nm light at room temperature in the presence of 3-(3',4'-dichlorophenyl)-1,1'-dimethylurea (DCMU) plus hydroxylamine, or by adjustment of the oxidation-reduction potential of the chloroplasts to - 150 mV prior to freezing. In the presence of ferricyanide (20-50 mM), two free-radical signals are photoinduced during a 660 nm flash at 35 degrees K. One signal decays with a half-time of 5 ms, whereas the second signal is formed irreversibly. These results are discussed in terms of a current model for the Photosystem II primary reaction at low temperature which postulates a back-reaction between P-680+ and the primary electron acceptor.
在低温下，用单色光进行激光闪光激活后，对光系统I和光系统II的初级反应物进行了电子顺磁共振研究。在35K的温度下，用730nm激光闪光激活后，叶绿体和光系统I片段中的P-700光氧化不可逆地发生。用660nm光对叶绿体或光系统II叶绿体片段进行闪光激活，会产生一个自由基信号（g = 2.002，线宽约8高斯），该信号在35K下以5.0ms的半衰期衰减。衰减的半衰期在10-77K范围内与温度无关。这个可逆信号可以通过在35K下用660nm光预照射样品（但不能用730nm光）、在室温下在3-(3',4'-二氯苯基)-1,1'-二甲基脲（DCMU）加羟胺存在下用660nm光预照射或在冷冻前将叶绿体的氧化还原电位调节到-150mV来消除。在铁氰化物（20-50mM）存在下，在35K的660nm闪光期间光诱导出两个自由基信号。一个信号以5ms的半衰期衰减，而第二个信号不可逆地形成。根据低温下光系统II初级反应的当前模型讨论了这些结果，该模型假设P-680+与初级电子受体之间存在反向反应。
