Dynamical structure of peptide molecules: Fourier transform microwave spectroscopy of N-methylpropionamide
Dynamical structure of peptide molecules: Fourier transform microwave spectroscopy of N-methylpropionamide
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DOI：
10.1016/s0022-2852(03)00026-2
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发表时间：
2003-05
影响因子：
1.4
通讯作者：
Y. Kawashima;R. Suenram;E. Hirota
中科院分区：
文献类型：
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作者：
Y. Kawashima;R. Suenram;E. Hirota
为了阐明肽结构的动力学方面，以 N-甲基丙酰胺 (NMPA) 作为肽分子的示例进行了研究：XCONHY（对于 NMPA，X=CH3CH2 和 Y=CH3），特别注意两个甲基的内旋转。发现 NMPA 具有几乎平面的骨架，具有扩展的顺式/反式构象，如 Iaa+Ibb−Icc 的观察值所示，其旋转光谱根据由六个对称物种组成的 G18 族进行解释：A1、A2、E1、E2、E3 和 E4。观察到 A1 和 E2 光谱在大多数 b 型跃迁中分裂，乙基基团（称为 C–CH3）中的 CH3 产生 796 (21)cm−1 的内旋转势垒 V3。三个 E 物种：E1、E3 和 E4 的光谱与相应的 A1 光谱相距数十至数千 MHz，表明与氮键合的 CH3（称为 N-CH3）的内旋转势垒相当低。与 A1 光谱形成鲜明对比的是，A1 光谱非常适合普通的非对称转子光谱模式，需要一些高阶项来重现 E1 光谱，大概是因为 N-CH3 势垒较低。事实上，由此进行的光谱分析得出的 V3 为 80.06487 (14)cm−1，比 C–CH3 低一个数量级。 E3和E4谱被发现与相应的E1谱线在中心形成三联体，并且E3-E1和E4-E1分裂基本上可以通过C-CH3内旋转与两个甲基之间的动能耦合的贡献来解释。从头计算表明，乙基和羰基之间的 C-C 键周围的扭转具有双极小性质，但观察到的 A1 谱没有显示出任何迹象表明 C-C 扭转存在这种双极小值，尽管平面构象中存在小驼峰的可能性不能完全消除。目前 NMPA 的结果以及其他肽分子的结果对于阐明结构生物学的重要问题（例如蛋白质折叠和通过生物系统的信号传递）具有一定的意义。
In order to clarify the dynamical aspects of the peptide structure, N-methylpropionamide (NMPA) was investigated as an example of peptide molecules: XCONHY (X=CH3CH2and Y=CH3for NMPA), paying special attention to the internal rotation of the two methyl groups. NMPA was found to have an almost planar skeleton with an extended syn/trans conformation, as indicated by the observed value of Iaa+Ibb−Icc, and its rotational spectra were interpreted in terms of group G18consisting of six symmetry species: A1, A2, E1, E2, E3, and E4. The A1and E2spectra were observed split in most of b-type transitions, yielding the internal-rotation potential barrier V3of 796 (21)cm−1for CH3in the ethyl group referred to as C–CH3. The spectra of the three E species: E1, E3, and E4appeared several tens to thousands MHz apart from the corresponding A1spectra, suggesting the internal-rotation potential barrier of CH3bonded to the nitrogen, called N–CH3, to be quite low. In sharp contrast with the A1spectra, which were well fitted to the ordinary asymmetric-rotor spectral pattern, a few higher-order terms were required to reproduce the E1spectra, presumably because of the low N–CH3barrier. The spectral analysis thus performed, in fact, led to the V3of 80.06487 (14)cm−1, an order of magnitude lower than that of C–CH3. The E3and E4spectra were found to form triplets with the corresponding E1lines at the center, and the E3–E1and E4–E1splittings were explained essentially by the contributions of the C–CH3internal rotation combined with the kinetic-energy coupling between the two methyl groups. The torsion around the C–C bond between the ethyl and carbonyl groups was suggested by an ab initio calculation to be of double minimum nature, but the observed A1spectra did not show any indication of such a double-minimum potential for the C–C torsion, although the possibility of a small hump being present at a planar conformation could not be entirely eliminated. The present results on NMPA along with those obtained on other peptide molecules will be of some significance in clarifying important problems of structural biology such as protein folding and signal transfer through biological systems.