Tait Claudia E, Stoll Stefan
Department of Chemistry, University of Washington, Seattle, WA 98195, USA.
Department of Chemistry, University of Washington, Seattle, WA 98195, USA.
J Magn Reson. 2017 Apr;277:36-44. doi: 10.1016/j.jmr.2017.02.007. Epub 2017 Feb 12.
Electron Nuclear DOuble Resonance (ENDOR) is based on the measurement of nuclear transition frequencies through detection of changes in the polarization of electron transitions. In Davies ENDOR, the initial polarization is generated by a selective microwave inversion pulse. The rectangular inversion pulses typically used are characterized by a relatively low selectivity, with full inversion achieved only for a limited number of spin packets with small resonance offsets. With the introduction of pulse shaping to EPR, the rectangular inversion pulses can be replaced with shaped pulses with increased selectivity. Band-selective inversion pulses are characterized by almost rectangular inversion profiles, leading to full inversion for spin packets with resonance offsets within the pulse excitation bandwidth and leaving spin packets outside the excitation bandwidth largely unaffected. Here, we explore the consequences of using different band-selective amplitude-modulated pulses designed for NMR as the inversion pulse in ENDOR. We find an increased sensitivity for small hyperfine couplings compared to rectangular pulses of the same bandwidth. In echo-detected Davies-type ENDOR, finite Fourier series inversion pulses combine the advantages of increased absolute ENDOR sensitivity of short rectangular inversion pulses and increased sensitivity for small hyperfine couplings of long rectangular inversion pulses. The use of pulses with an almost rectangular frequency-domain profile also allows for increased control of the hyperfine contrast selectivity. At X-band, acquisition of echo transients as a function of radiofrequency and appropriate selection of integration windows during data processing allows efficient separation of contributions from weakly and strongly coupled nuclei in overlapping ENDOR spectra within a single experiment.
电子-核双共振（ENDOR）基于通过检测电子跃迁极化变化来测量核跃迁频率。在戴维斯ENDOR中，初始极化由选择性微波反转脉冲产生。通常使用的矩形反转脉冲具有相对较低的选择性，仅对有限数量的具有小共振偏移的自旋包实现完全反转。随着脉冲整形技术引入电子顺磁共振（EPR），矩形反转脉冲可被选择性更高的整形脉冲取代。带选择性反转脉冲的特点是反转轮廓几乎为矩形，导致脉冲激发带宽内具有共振偏移的自旋包完全反转，而激发带宽外的自旋包基本不受影响。在此，我们探讨将为核磁共振（NMR）设计的不同带选择性幅度调制脉冲用作ENDOR中的反转脉冲的后果。我们发现，与相同带宽的矩形脉冲相比，对于小超精细耦合，灵敏度有所提高。在回波检测的戴维斯型ENDOR中，有限傅里叶级数反转脉冲兼具短矩形反转脉冲绝对ENDOR灵敏度提高和长矩形反转脉冲对小超精细耦合灵敏度提高的优点。使用频域轮廓几乎为矩形的脉冲还能增强对超精细对比度选择性的控制。在X波段，采集作为射频函数的回波瞬态，并在数据处理期间适当选择积分窗口，能够在单个实验中有效分离重叠ENDOR谱中弱耦合和强耦合核的贡献。
