Institute of Physical and Theoretical Chemistry, University of Bonn, 53115, Bonn, Germany.
Current address: Institute of Inorganic Chemistry, University of Bonn, 53115, Bonn, Germany.
Chemistry. 2019 Nov 13;25(63):14388-14398. doi: 10.1002/chem.201902908. Epub 2019 Oct 9.
Pulsed electron paramagnetic resonance (EPR) dipolar spectroscopy (PDS) offers several methods for measuring dipolar coupling constants and thus the distance between electron spin centers. Up to now, PDS measurements have been mostly applied to spin centers whose g-anisotropies are moderate and therefore have a negligible effect on the dipolar coupling constants. In contrast, spin centers with large g-anisotropy yield dipolar coupling constants that depend on the g-values. In this case, the usual methods of extracting distances from the raw PDS data cannot be applied. Here, the effect of the g-anisotropy on PDS data is studied in detail on the example of the low-spin Fe ion. First, this effect is described theoretically, using the work of Bedilo and Maryasov (Appl. Magn. Reson. 2006, 30, 683-702) as a basis. Then, two known Fe /nitroxide compounds and one new Fe /trityl compound were synthesized and PDS measurements were carried out on them using a method called relaxation induced dipolar modulation enhancement (RIDME). Based on the theoretical results, a RIDME data analysis procedure was developed, which facilitated the extraction of the inter-spin distance and the orientation of the inter-spin vector relative to the Fe g-tensor frame from the RIDME data. The accuracy of the determined distances and orientations was confirmed by comparison with MD simulations. This method can thus be applied to the highly relevant class of metalloproteins with, for example, low-spin Fe ions.
脉冲电子顺磁共振（EPR）偶极子光谱（PDS）提供了几种测量偶极子耦合常数的方法，从而可以确定电子自旋中心之间的距离。到目前为止，PDS 测量主要应用于 g 各向异性适中的自旋中心，因此对偶极子耦合常数的影响可以忽略不计。相比之下，具有较大各向异性的自旋中心产生的偶极子耦合常数取决于 g 值。在这种情况下，不能从原始 PDS 数据中应用通常的距离提取方法。在这里，以低自旋 Fe 离子为例，详细研究了 g 各向异性对 PDS 数据的影响。首先，使用 Bedilo 和 Maryasov 的工作（Appl. Magn. Reson. 2006, 30, 683-702）作为基础，从理论上描述了这种影响。然后，合成了两种已知的 Fe/氮氧自由基化合物和一种新的 Fe/三苯甲基化合物，并使用称为弛豫诱导偶极调制增强（RIDME）的方法对它们进行了 PDS 测量。基于理论结果，开发了一种 RIDME 数据分析程序，该程序便于从 RIDME 数据中提取自旋间距离和自旋矢量相对于 Fe g-张量框架的方向。通过与 MD 模拟的比较，证实了所确定的距离和取向的准确性。因此，该方法可应用于具有低自旋 Fe 离子的高度相关的金属蛋白类。
