Uncoupled adjacent amide vibrations in small peptides
Uncoupled adjacent amide vibrations in small peptides
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DOI：
10.1021/ja0004783
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发表时间：
2000-09-20
影响因子：
15
通讯作者：
Asher, SA
中科院分区：
文献类型：
--
作者：
Mix, G;Schweitzer-Stenner, R;Asher, SA
振动光谱学是用于检查分子结构和动力学的经典技术。[1]主要的挑战通常是指定振动模式，并根据分子坐标解释它们的频率和强度。如果发生与感兴趣的分子坐标相关联的群振动，则这通常被简化。2聚合物中的官能团振动可以耦合，如果它们彼此靠近，以产生集体聚合物振动。耦合振动的观察在红外光谱中应该更常见，因为红外选择具有大的偶极矩变化的振动。具有大的偶极矩变化的振动可以通过跃迁偶极耦合通过空间耦合。与此相反，共振拉曼光谱（RR）选择通过独立的偶极矩变化的标准。因此，RR观测到的振动不太可能耦合。在这里，我们研究酰胺振动之间的耦合在三肽和衍生物与相邻的酰胺基团，并询问是否由RR观察到的振动结果从振动本地化在个别酰胺肽键，或者是否这些振动是离域和相邻的酰胺肽键的耦合运动的结果。这项工作是研究计划的一部分，我们正在开发UVRR用于研究生物结构和功能。3我们最近证明，酰胺π f π* 跃迁内的RR激发增强了酰胺R带。4产生的酰胺RR光谱定量确定肽和蛋白质二级结构。5，6此外，我们已经使用UVRR来探测由于ns温度跳变引起的肽折叠和解折叠的第一步。7，8我们通过测量连接酰胺的RR光谱来确定酰胺基团之间的振动耦合程度。我们比较了天然同位素异构体的光谱，那些通过交换不稳定的NH基团在D2 O中的ND。然后，我们测量这些衍生物在混合的H2O/D2 O溶液中，其中NH基团仅部分氘代。用ND取代NH会显著改变正常模式，因为NH运动不再与CN运动耦合。假设如下：如果连接的酰胺的简正模偶联，则酰胺的氘代将干扰连接的非氘代酰胺的频率和RR截面。因此，UVRR不能被建模为纯氘代和非氘代衍生物的总和。9图1显示了连接的酰胺衍生物N-乙酰基-N '-甲基甘氨酰胺（AcGNMe）、N-乙酰基-N'-甲基-L-丙氨酰胺（AcANMe）、三甘氨酸（G3）和三丙氨酸（A3）在水和纯D2 O中的RR光谱。我们观察到一个更复杂的光谱在水，其中连接的酰胺给重叠的AmI，AmII，和AmIII带。AmII和AmIII带的特征描述为涉及耦合CN拉伸和NH面内弯曲。然而，正如其他人所指出的，AmIII振动具有更复杂的组成，这取决于确切的分子结构。10-12在D2 O中，由于AmII和III模消失，并被非常强的AmII′模取代，这主要是CN伸缩，因此光谱大大简化。[13]氘代衍生物的AmI′模相对于氢化衍生物的AmI′模发生了位移。因此，NH光谱和ND光谱显著不同。
Vibrational spectroscopy is a classical technique used to examine molecular structure and dynamics. 1 The major challenge often is assigning vibrational modes and interpreting their frequencies and intensities in terms of the molecular coordinates. This is often simplified if group vibrations occur which are associated with molecular coordinates of interest. 2 Functional group vibrations within polymers can couple if they are located close to one another to give collective polymer vibrations. The observation of coupled vibrations should be more common in IR spectroscopy because IR selects for vibrations with large dipole moment changes. Vibrations with large dipole moment changes can couple through space by transition dipole coupling. In contrast, resonance Raman spectroscopy (RR) selects through criteria independent of dipole moment changes. Thus, vibrations observed by RR are less likely to be coupled. Here we examine coupling between amide vibrations in tripeptides and in derivatives with adjacent amide groups and ask whether the vibrations observed by RR result from vibrations localized within individual amide peptide bonds, or whether these vibrations are delocalized and result from coupled motion of adjacent amide peptide bonds. This work is part of a research program where we are developing UVRR for studying biological structure and function. 3 We recently demonstrated that RR excitation within the amide π f π* transitions enhanced amide R bands. 4 The resulting amide RR spectra quantitatively determine peptide and protein secondary structure. 5, 6 In addition, we have used UVRR to probe the first steps in the folding and unfolding of peptides due to ns temperature jumps. 7, 8 We determined the extent of vibrational coupling between amide groups by measuring the RR spectra of linked amides. We compared the spectra of natural isotopomers to those formed by exchanging the labile NH groups by ND in D2O. We then measured these derivatives in a mixed H2O/D2O solution, where NH groups were only partially deuterated. Replacement of NH by ND dramatically alters the normal mode, since NH motion no longer couples to CN motion. The hypothesis follows: if normal modes of the linked amides couple, deuteration of an amide would perturb the frequencies and RR cross sections of the linked nondeuterated amide. Thus, UVRR could not be modeled as a sum of pure deuterated and nondeuterated derivatives. 9 Figure 1 shows the RR spectra of the linked amide derivatives N-acetyl-N′-methylglycinamide (AcGNMe), N-acetyl-N′-methyl-L-alanylamide (AcANMe), triglycine (G3), and trialanine (A3) in water and pure D2O. We observe a more complex spectrum in H2O, where the linked amides give overlapping AmI, AmII, and AmIII bands. The AmII and AmIII bands are characteristically described as involving coupled CN stretching and NH inplane bending. However, as noted by others, the AmIII vibration has a more complex composition which depends on the exact molecular structure. 10-12The spectra considerably simplify in D2O because the AmII and III modes disappear and are replaced by very intense AmII′ modes, which are mainly CN stretching. 13 The AmI′ modes of the deuterated derivatives shift relative to those of the hydrogenated derivative. Thus, the NH spectra and ND spectra differ dramatically.