In this review on advanced EPR spectroscopy, which addresses both the EPR and NMR communities, considerable emphasis is put on delineating the complementarity of NMR and EPR concerning the measurement of molecular interactions in large biomolecules. From these interactions, detailed information can be revealed on structure and dynamics of macromolecules embedded in solution- or solid-state environments. New developments in pulsed microwave and sweepable cryomagnet technology as well as ultrafast electronics for signal data handling and processing have pushed to new horizons the limits of EPR spectroscopy and its multifrequency extensions concerning the sensitivity of detection, the selectivity with respect to interactions, and the resolution in frequency and time domains. One of the most important advances has been the extension of EPR to high magnetic fields and microwave frequencies, very much in analogy to what happens in NMR. This is exemplified by referring to ongoing efforts for signal enhancement in both NMR and EPR double-resonance techniques by exploiting dynamic nuclear or electron spin polarization via unpaired electron spins and their electron-nuclear or electron-electron interactions. Signal and resolution enhancements are particularly spectacular for double-resonance techniques such as ENDOR and PELDOR at high magnetic fields. They provide greatly improved orientational selection for disordered samples that approaches single-crystal resolution at canonical g-tensor orientations - even for molecules with small g-anisotropies. Exchange of experience between the EPR and NMR communities allows for handling polarization and resolution improvement strategies in an optimal manner. Consequently, a dramatic improvement of EPR detection sensitivity could be achieved, even for short-lived paramagnetic reaction intermediates. Unique structural and dynamic information is thus revealed that can hardly be obtained by any other analytical techniques. Micromolar quantities of sample molecules have become sufficient to characterize stable and transient reaction intermediates of complex molecular systems - offering highly interesting applications for chemists, biochemists and molecular biologists. In three case studies, representative examples of advanced EPR spectroscopy are reviewed: (I) High-field PELDOR and ENDOR structure determination of cation-anion radical pairs in reaction centers from photosynthetic purple bacteria and cyanobacteria (Photosystem I); (II) High-field ENDOR and ELDOR-detected NMR spectroscopy on the oxygen-evolving complex of Photosystem II; and (III) High-field electron dipolar spectroscopy on nitroxide spin-labelled bacteriorhodopsin for structure-function studies. An extended conclusion with an outlook to further developments and applications is also presented.

中文翻译：

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膜蛋白上的高场 EPR——跨越 NMR 的鸿沟

在这篇关于 EPR 和 NMR 社区的高级 EPR 光谱综述中，相当重视描述 NMR 和 EPR 在测量大生物分子中的分子相互作用方面的互补性。从这些相互作用中，可以揭示嵌入在溶液或固态环境中的大分子的结构和动力学的详细信息。脉冲微波和可扫描低温磁体技术以及用于信号数据处理和处理的超快电子学的新发展将 EPR 光谱的极限及其在检测灵敏度、相互作用方面的选择性和分辨率方面的多频扩展推向了新的境界在频域和时域。最重要的进步之一是将 EPR 扩展到高磁场和微波频率，这与 NMR 中发生的情况非常相似。这可以通过参考通过不成对电子自旋及其电子 - 核或电子 - 电子相互作用利用动态核或电子自旋极化来增强 NMR 和 EPR 双共振技术中信号的持续努力来说明。信号和分辨率增强对于高磁场下的双共振技术（如 ENDOR 和 PELDOR）尤其引人注目。它们为无序样品提供了极大改进的取向选择，在规范的 g-张量方向上接近单晶分辨率 - 即使对于具有小 g-各向异性的分子也是如此。EPR 和 NMR 社区之间的经验交流允许以最佳方式处理极化和分辨率改进策略。因此，即使对于短寿命的顺磁性反应中间体，也可以实现 EPR 检测灵敏度的显着提高。从而揭示了任何其他分析技术几乎无法获得的独特结构和动态信息。微摩尔量的样品分子已经足以表征复杂分子系统的稳定和瞬态反应中间体——为化学家、生物化学家和分子生物学家提供了非常有趣的应用。在三个案例研究中，回顾了高级 EPR 光谱的代表性示例：(I) 光合紫色细菌和蓝细菌（Photosystem I）反应中心阳离子-阴离子自由基对的高场PELDOR和ENDOR结构测定；(II) 光系统 II 的析氧复合物的高场 ENDOR 和 ELDOR 检测核磁共振光谱；(III) 用于结构-功能研究的氮氧化物自旋标记的细菌视紫红质的高场电子偶极光谱。还提出了一个扩展结论，展望了进一步的发展和应用。