Schlamadinger Diana E, Daschbach Megan M, Gokel George W, Kim Judy E
Department of Chemistry and Biochemistry, University of California at San Diego, La Jolla, CA 92093 USA.
Department of Chemistry, Washington University, St. Louis, MO 63110 USA.
J Raman Spectrosc. 2011 Apr;42(4):633-638. doi: 10.1002/jrs.2781.
UV resonance Raman (UVRR) spectroscopy is used to probe changes in vibrational structure associated with cation-π interactions for the most prevalent amino acid π -donor, tryptophan. The model compound studied here is a diaza crown ether with two indole substituents. In the presence of sodium or potassium sequestered in the crown ether, or a protonated diaza group on the compound, the indole moieties participate in a cation-π interaction in which the pyrrolo group acts as the primary π-donor. Systematic shifts in relative intensity in the 760-780 cm region are observed upon formation of this cation-π interaction; we propose that these modifications reflect shifts of the delocalized, ring-breathing W18 and hydrogen-out-of-plane (HOOP) vibrational modes in this spectral region. The observed changes are attributed to perturbations of the π-electron density as well as of normal modes that involve large displacement of the hydrogen atom on the C2 position of the pyrrole ring. Modest variations in the UVRR spectra for the three complexes studied here are correlated to differences in cation-π strength. Specifically, the UVRR spectrum of the sodium-bound complex differs from those of the potassium-bound or protonated-diaza complexes, and may reflect the observation that the C2 hydrogen atom in the sodium-bound complex exhibits the greatest perturbation relative to the other species. Normal modes sensitive to hydrogen-bonding, such as the tryptophan W10, W9, and W8 modes, also undergo shifts in the presence of the salts. These shifts reflect the strength of interaction of the indole N-H group with the iodide or hexafluorophosphate counteranion. The current observation that the W18 and HOOP normal mode regions of the indole crown ether compound are sensitive to cation-pyrrolo π interactions suggests that this region may provide reliable spectroscopic evidence of these important interactions in proteins.
紫外共振拉曼（UVRR）光谱用于探测与最常见的氨基酸π供体色氨酸的阳离子-π相互作用相关的振动结构变化。这里研究的模型化合物是一种带有两个吲哚取代基的二氮杂冠醚。在冠醚中螯合有钠或钾，或者化合物上有质子化的二氮杂基团的情况下，吲哚部分参与阳离子-π相互作用，其中吡咯基团作为主要的π供体。在形成这种阳离子-π相互作用时，在760 - 780厘米区域观察到相对强度的系统变化；我们认为这些变化反映了该光谱区域中离域的、环呼吸W18和氢面外（HOOP）振动模式的位移。观察到的变化归因于π电子密度的扰动以及涉及吡咯环C2位置上氢原子大位移的简正模式的扰动。这里研究的三种配合物的UVRR光谱的适度变化与阳离子-π强度的差异相关。具体而言，钠结合配合物的UVRR光谱与钾结合或质子化二氮杂配合物的光谱不同，并且可能反映了这样的观察结果，即钠结合配合物中的C2氢原子相对于其他物种表现出最大的扰动。对氢键敏感的简正模式，如色氨酸的W10、W9和W8模式，在盐存在时也会发生位移。这些位移反映了吲哚N - H基团与碘化物或六氟磷酸根抗衡离子的相互作用强度。目前观察到吲哚冠醚化合物的W18和HOOP简正模式区域对阳离子-吡咯π相互作用敏感，这表明该区域可能为蛋白质中这些重要相互作用提供可靠的光谱证据。
