Two-dimensional infrared spectroscopy of the alanine dipeptide in aqueous solution
Two-dimensional infrared spectroscopy of the alanine dipeptide in aqueous solution
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DOI：
10.1021/jp044989d
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发表时间：
2005-04-21
影响因子：
3.3
通讯作者：
Hochstrasser, RM
中科院分区：
文献类型：
--
作者：
Kim, YS;Wang, JP;Hochstrasser, RM
本文报道了丙氨酸二肽及其C-13同位素在水溶液（D_2O）中酰胺-I ′区的线性红外和二维红外光谱。通过使用羰基的C-13同位素取代，已经明确地分配了两个酰胺-I ' IR跃迁;在乙酰基末端的酰胺单元在未标记的物质中显示出较低的跃迁频率。它们的跃迁偶极强度之比在C-13取代后几乎保持不变，表明两个振子之间没有强度转移。在这种情况下，与模内耦合直接相关的2D IR交叉峰显示出小的非对角非谐性（0.2 +/-0.2 cm（-1）），导致小的耦合常数（1.5 +/-0.5 cm（-1））。在两个不同偏振（和）下的耦合和2D IR光谱< zzzz >< zxxz >如对于二丙氨酸的聚脯氨酸-II（PPII）样构象所预期的那样，主链二面角（phi，psi）被确定为在（-70度+/-25度，+ 120度+/-25度）的范围内。从头算DFT计算和正常模式的解耦分析的Ramachandran子空间在附近的PPII构象确认存在的耦合是vanishingly小的区域，并支持这些实验结果。耦合和非对角非谐性之间的关系是通过检查后者的分布，从一个集合平均的哈密顿量，包括不相关的对角频率分布和一个小的耦合（< 2 cm（-1）），它被发现，最可能的值为非对角非谐性福尔斯落在实验观测的范围内。此外，结合DFT结果，模拟的线性IR和二维IR可以再现测量的基本特征，包括跃迁频率位置和表观峰强度。所有的实验结果和模拟是一致的PPII样构象的丙氨酸二肽在水溶液中，其中两个酰胺-I '模式是高度本地化的，其频率分布是不相关的。
The linear-infrared and two-dimensional infrared (2D IR) spectra in the amide-I' region of the alanine dipeptide and its C-13 isotopomers in aqueous solution (D2O) are reported. The two amide-I' IR transitions have been assigned unambiguously by using C-13 isotopic substitution of the carbonyl group; the amide unit at the acetyl end shows a lower transition frequency in the unlabeled species. The ratio of their transition dipole strengths remains almost unchanged upon C-13 substitution, indicating the absence of intensity transfer between two vibrators. The 2D IR cross peaks directly associated with intramode coupling in this case show a small off-diagonal anharmonicity (0.2 +/- 0.2 cm(-1)), leading to a small coupling constant (1.5 +/- 0.5 cm(-1)). The coupling and the 2D IR spectra in two different polarizations (< zzzz > and < zxxz >) are as expected for a polyproline-II (PPII)-like conformation for dialanine, with the backbone dihedral angles (phi, psi) determined to be in the range of (-70 degrees +/- 25 degrees, + 120 degrees +/- 25 degrees). Ab initio DFT calculations and normal mode decoupling analysis in the Ramachandran subspace in the neighborhood of PPII conformation confirm the presence of a region where the coupling is vanishingly small and support these experimental findings. The relationship between the coupling and off-diagonal anharmonicity is consolidated by examining the distribution of the latter from an ensemble averaged Hamiltonian incorporating uncorrelated diagonal frequency distributions and a small coupling (< 2 cm(-1)); it is found that the most probable value for the off-diagonal anharmonicity falls into the range of experimental observations. Further, incorporating DFT results, the simulated linear-IR and 2D IR can reproduce the essential features of the measurements, including the transition frequency positions and apparent peak intensities. All the experimental results and simulations are consistent with a PPII-Iike conformation for the alanine dipeptide in aqueous solution, in which two amide-I' modes are highly localized and whose frequency distributions are uncorrelated.