Stropp Julian, Wili Nino, Nielsen Niels C, Klose Daniel
Institute for Molecular Physical Science, ETH Zurich, Vladimir-Prelog-Weg 2, 8093 Zurich, Switzerland.
Interdisciplinary Nanoscience Center and Department of Chemistry, Aarhus University, Gustav Wieds Vej 14, 8000 Aarhus C, Denmark.
Magn Reson (Gott). 2025 Jan 24;6(1):33-42. doi: 10.5194/mr-6-33-2025. eCollection 2025.
Electron-nuclear double resonance (ENDOR) spectroscopy is an EPR technique to detect the nuclear frequency spectra of hyperfine coupled nuclei close to paramagnetic centers, which have interactions that are not resolved in continuous wave EPR spectra and may be fast relaxing on the timescale of NMR. For the common case of non-crystalline solids, such as powders or frozen solutions of transition metal complexes, the anisotropy of the hyperfine and nuclear quadrupole interactions renders ENDOR lines often several megahertz (MHz) broad, thus diminishing intensity. With commonly used ENDOR pulse sequences, only a small fraction of the NMR/ENDOR line is excited with a typical radiofrequency (RF) pulse length of several tens of microseconds ( s), and this limits the sensitivity in conventional ENDOR experiments. In this work, we show the benefit of chirped RF excitation in frequency-domain ENDOR as a simple yet effective way to significantly improve sensitivity. We demonstrate on a frozen solution of Cu(II)-tetraphenylporphyrin that the intensity of broad copper and nitrogen ENDOR lines increases up to 9-fold compared to single-frequency RF excitation, thus making the detection of metal ENDOR spectra more feasible. The tunable bandwidth of the chirp RF pulses allows the operator to optimize for sensitivity and choose a tradeoff with resolution, opening up options previously inaccessible in ENDOR spectroscopy. Also, chirp pulses help to reduce RF amplifier overtones, since lower RF powers suffice to achieve intensities matching conventional ENDOR. In 2D triple resonance experiments (TRIPLE), the signal increase exceeds 10 times for some lines, thus making chirped 2D TRIPLE experiments feasible even for broad peaks in manageable acquisition times.
电子-核双共振（ENDOR）光谱学是一种电子顺磁共振（EPR）技术，用于检测靠近顺磁中心的超精细耦合核的核频谱，这些核的相互作用在连续波EPR光谱中无法分辨，并且在核磁共振（NMR）的时间尺度上可能快速弛豫。对于非晶态固体的常见情况，如过渡金属配合物的粉末或冷冻溶液，超精细和核四极相互作用的各向异性使得ENDOR谱线通常宽达几兆赫兹（MHz），从而降低了强度。使用常用的ENDOR脉冲序列，在典型的几十微秒（μs）射频（RF）脉冲长度下，只有一小部分NMR/ENDOR谱线被激发，这限制了传统ENDOR实验的灵敏度。在这项工作中，我们展示了频域ENDOR中啁啾RF激发的优势，这是一种显著提高灵敏度的简单而有效的方法。我们在Cu（II）-四苯基卟啉的冷冻溶液上证明，与单频RF激发相比，宽的铜和氮ENDOR谱线的强度增加了9倍，从而使金属ENDOR光谱的检测更加可行。啁啾RF脉冲的可调带宽使操作人员能够优化灵敏度，并在灵敏度和分辨率之间进行权衡，开辟了以前在ENDOR光谱学中无法实现的选择。此外，啁啾脉冲有助于减少RF放大器的谐波，因为较低的RF功率就足以达到与传统ENDOR相匹配的强度。在二维三重共振实验（TRIPLE）中，某些谱线的信号增加超过10倍，从而使啁啾二维TRIPLE实验即使对于宽峰也能在可管理的采集时间内可行。
