Center for Molecular Spectroscopy and Dynamics, Institute for Basic Science (IBS), Korea University, Seoul 02841, Republic of Korea.
Department of Chemistry, Korea University, Seoul 02841, Republic of Korea.
Phys Chem Chem Phys. 2020 Sep 8;22(34):19223-19229. doi: 10.1039/d0cp02693j.
Azido stretch modes in a variety of azido-derivatized nonnatural amino acids and nucleotides have been used as a site-specific infrared (IR) probe for monitoring changes in their conformations and local electrostatic environments. The vibrational bands of azide probes are often accompanied by complex line shapes with shoulder peaks, which may arise either from incomplete background subtraction, Fermi resonance, or multiple conformers. The isotope substitution in the infrared probe has thus been introduced to remove Fermi resonances without causing a significant perturbation to the structure. Here, we synthesized and labeled the mid-N atoms of aliphatic azide derivatives with 15N to study the effects of isotope labelling on their vibrational properties. The FT-IR spectra of the aliphatic azide with asymmetric lineshape became a single symmetric band upon isotope substitution, which might be an indication of the removal of the hidden Fermi resonance from the system. We also noticed that the 2D-IR spectrum of unlabeled aliphatic azide has cross-peaks, even though it is not apparently identifiable. The 1D slice spectra obtained from the 2D-IR spectra reveal the existence of a hidden Fermi resonance peak. Furthermore, we show that this weak Fermi resonance does not produce discernible oscillatory beating patterns in the IR pump-probe spectrum, which has been used as evidence of the Fermi resonance. Therefore, we confirm that isotope labelling combined with 2D-IR spectroscopy is the most efficient and incisive way to identify the origin of small shoulder peaks in the linear and nonlinear vibrational spectra of various IR probe molecules.
各种叠氮化物衍生的非天然氨基酸和核苷酸中的叠氮伸缩模式已被用作监测其构象和局部静电环境变化的特定于位置的红外（IR）探针。叠氮探针的振动带通常伴随着具有肩峰的复杂线形状，这些肩峰可能是由于不完全的背景扣除、费米共振或多个构象体引起的。因此，已经引入了红外探针中的同位素取代，以消除费米共振而不对结构造成显著干扰。在这里，我们合成并标记了脂肪族叠氮衍生物的中 N 原子的 15N，以研究同位素标记对其振动特性的影响。具有不对称线形状的脂肪族叠氮的 FT-IR 光谱在同位素取代后变为单个对称带，这可能表明系统中隐藏的费米共振已被消除。我们还注意到，即使未标记的脂肪族叠氮的 2D-IR 光谱没有明显的可识别性，也存在交叉峰。从 2D-IR 光谱获得的 1D 切片光谱显示存在隐藏的费米共振峰。此外，我们表明，这种弱费米共振不会在 IR 泵浦探测光谱中产生可辨别的振荡拍频图案，这已被用作费米共振的证据。因此，我们确认，同位素标记结合 2D-IR 光谱是识别各种 IR 探针分子的线性和非线性振动光谱中小肩峰的最有效和敏锐的方法。
