In this work, we investigated the redox transients of a number of water-soluble spin labels upon their interactions with Photosystem II (PS II) core complexes isolated from spinach leaves. We have found that the reactivity of nitroxide radicals, determined by the rate of their reduction upon illumination of PS II, depends on the chemical structure of radicals and the capability of their coming close to low-potential redox centers of photoactive PS II complexes. An enhanced capability of nitroxide radicals to accept electrons from PS II correlates with their chemical structure. Nitroxide radicals NTI (2,2,5,5-tetramethyl-4-nitromethylene-3-imidazolidine-N-oxyl) and Tacet (4-hydroxy-2,2,6,6-tetramethylpiperidine-1-oxyl-acetate), containing polar groups, appear to be most efficient acceptors of electrons donated by PS II compared to neutral (TEMPOL, 4-hydroxy-2,2,6,6-tetramethylpiperidine-1-oxyl) or positively charged (Tamine, 4-amino-2,2,6,6-tetramethylpiperidine-l-oxyl) spin labels. We assume that enhanced reactivities of polar nitroxide radicals, NTI and Tacet, are determined (1) by their relatively high redox potentials, providing the possibility to accept electrons from PS II, and (2) by their affinities to the closest binding sites on the surface of PS II in the vicinity of the primary plastoquinone acceptor PQ_A (12–14 Å) or/and in the intraprotein cavity for the secondary plastoquinone PQ_B (~ 22 Å).

在这项工作中，我们研究了许多水溶性自旋标签与从菠菜叶中分离出来的光系统 II (PS II) 核心复合物相互作用时的氧化还原瞬变。我们发现，氮氧自由基的反应性，取决于它们在 PS II 照射下的还原速率，取决于自由基的化学结构和它们接近光活性 PS II 配合物的低电位氧化还原中心的能力。氮氧自由基接受来自 PS II 的电子的能力增强与其化学结构相关。氮氧自由基 NTI (2,2,5,5-tetramethyl-4-nitromethylene-3-imidazolidine- N-oxyl) 和 Tacet (4-hydroxy-2,2,6,6-tetramethylpiperidine-1-oxyl-acetate)，与中性 (TEMPOL, 4 -hydroxy-2,2,6,6-tetramethylpiperidine-1-oxyl) 或带正电荷的 (Tamine, 4-amino-2,2,6,6-tetramethylpiperidine-1-oxyl) 自旋标签。我们假设极性氮氧自由基 NTI 和 Tacet 的增强反应性是由（1）它们相对较高的氧化还原电位决定的，这提供了从 PS II 接受电子的可能性，以及（2）它们对最近的结合位点的亲和力。 PS II 的表面在初级质体醌受体 PQ _A (12-14 Å) 附近或/和在次级质体醌 PQ _B (~ 22 Å) 的蛋白内腔中。