Ritsch Irina, Hintz Henrik, Jeschke Gunnar, Godt Adelheid, Yulikov Maxim
Laboratory of Physical Chemistry, Department of Chemistry and Applied Bioscience, ETH Zurich, Vladimir-Prelogweg 2, 8093 Zurich, Switzerland.
Phys Chem Chem Phys. 2019 May 15;21(19):9810-9830. doi: 10.1039/c8cp06573j.
Orientation selection is a challenge in distance determination with double electron electron resonance (DEER) spectroscopy of rigid molecules. The problem is reduced when applying the Relaxation-Induced Dipolar Modulation Enhancement (RIDME) experiment. Here we present an in-depth study on nitroxide-detected RIDME in Cu(ii)-nitroxide spin pairs using two Cu(ii)-nitroxide rulers that are both water soluble and have comparable spin-spin distances. They differ in the type of the ligand (TAHA and PyMTA) for the Cu(ii) ion which results in different contributions of exchange coupling. Both rulers feature substantial orientation correlation between the molecular frames of the Cu(ii) complex and the nitroxide. We discuss how the spin-spin couplings can be accurately measured and how they can be correlated to the nitroxide resonance frequencies. In that, we pay particular attention to the suppression of nuclear modulation and of echo crossing artefacts, to background correction, and to orientation averaging. With a nitroxide observer sequence based on chirp pulses, we achieve wideband detection of all nitroxide orientations. Two-dimensional Fourier transformation of data obtained in this manner affords observer-EPR correlated RIDME spectra that enable visual understanding of the orientation correlation. The syntheses of the Cu(ii)-nitroxide rulers are presented. The synthetic route is considered to be of general use for the preparation of [metal ion complex]-nitroxide rulers, including water soluble ones.
在对刚性分子进行双电子电子共振（DEER）光谱距离测定时，取向选择是一项挑战。应用弛豫诱导偶极调制增强（RIDME）实验可减少该问题。本文我们使用两种均为水溶性且具有可比自旋 - 自旋距离的铜（II） - 氮氧化物尺子，对铜（II） - 氮氧化物自旋对中氮氧化物检测的RIDME进行了深入研究。它们在铜（II）离子的配体类型（TAHA和PyMTA）上有所不同，这导致交换耦合的贡献不同。两种尺子在铜（II）配合物和氮氧化物的分子框架之间都具有显著的取向相关性。我们讨论了如何准确测量自旋 - 自旋耦合以及它们如何与氮氧化物共振频率相关联。在此过程中，我们特别关注核调制和回波交叉伪影的抑制、背景校正以及取向平均。通过基于啁啾脉冲的氮氧化物观测序列，我们实现了对所有氮氧化物取向的宽带检测。以这种方式获得的数据进行二维傅里叶变换，可得到观测者 - EPR相关的RIDME光谱，从而能够直观理解取向相关性。本文还介绍了铜（II） - 氮氧化物尺子的合成方法。该合成路线被认为对制备[金属离子配合物] - 氮氧化物尺子（包括水溶性尺子）具有普遍适用性。
