Pulsed electron paramagnetic resonance (EPR) spectroscopy using microwaves at two frequencies can be employed to measure distances between pairs of paramagnets separated by up to 10 nm. The method, combined with site-directed mutagenesis, has become increasingly popular in structural biology for both its selectivity and capability of providing information not accessible through more standard methods such as nuclear magnetic resonance and X-ray crystallography. Despite these advantages, EPR distance measurements suffer from poor sensitivity. One contributing factor is technical: since 65 MHz typically separates the pump and detection frequencies, they cannot both be located at the center of the pseudo-Lorentzian microwave resonance of a single-mode resonator. To maximize the inversion efficiency, the pump pulse is usually placed at the center of the resonance, while the observer frequency is placed in the wing, with consequent reduction in sensitivity. Here, we consider an alternative configuration: by spacing pump and observer frequencies symmetrically with respect to the microwave resonance and by increasing the quality factor, valuable improvement in the signal-to-noise ratio can be obtained.

中文翻译：

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利用谐振腔模式的对称性来增强 PELDOR 灵敏度

使用两个频率的微波的脉冲电子顺磁共振 (EPR) 光谱可用于测量相距达 10 nm 的顺磁体对之间的距离。该方法与定点诱变相结合，因其选择性和提供无法通过更标准方法（如核磁共振和 X 射线晶体学）获得的信息的能力而在结构生物学中变得越来越流行。尽管有这些优点，但 EPR 距离测量的灵敏度很差。一个促成因素是技术性的：由于 65 MHz 通常将泵浦频率和检测频率分开，因此它们不能同时位于单模谐振器的伪洛伦兹微波谐振的中心。为了最大化反演效率，泵浦脉冲通常位于共振的中心，而观察者频率位于机翼，因此灵敏度会降低。在这里，我们考虑另一种配置：通过相对于微波共振对称地间隔泵和观测器频率，并通过增加品质因数，可以获得有价值的信噪比改进。