Raman optical activity (ROA) spectroscopy combined with quantum-chemical simulations is a sensitive method to determine the absolute configuration and conformation of chiral molecules in solutions. However, the precision of this approach varies for different systems. In the present study, the reliability and numerical stability of decomposing experimental spectra into calculated subspectra is tested on the Ala-Ala dipeptide. Molecular dynamics (MD) snapshots of Ala-Ala/water clusters are averaged to account for solvent effects and molecular flexibility. Multiple experiments with protonated, zwitterionic, and deprotonated dipeptide forms and natural and d₂- and d₈-isotopically labeled dipeptides are used to verify the results and estimate the overall accuracy. Although the precision is still limited by experimental noise and computational error, a very close match between the observed and theoretical spectral shapes has been achieved. This enabled quantitative determination of conformer populations with a typical dispersion of 10%. The spectroscopy also demonstrated how the conformation depends on pH. The ROA results were more consistent than the Raman ones. Typically, the ROA analysis was more resistant to artifacts in the experiment, such as incomplete baseline subtraction. Conformer ratios predicted by MD agree fairly but not fully with the experimental ones. This indicates minor deficiencies in the Amber force field, particularly for the protonated dipeptide. Overall, the combination of ROA experiment and computational chemistry appears to be a robust tool providing deep insight into molecular structure.

中文翻译：

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拉曼光学活性谱的分解定量测定溶液中丙氨酸-丙氨酸构象比

拉曼光学活性（ROA）光谱与量子化学模拟相结合是确定溶液中手性分子的绝对构型和构象的灵敏方法。但是，此方法的精度因不同的系统而异。在本研究中，在Ala-Ala二肽上测试了将实验光谱分解为计算的子光谱的可靠性和数值稳定性。将Ala-Ala /水簇的分子动力学（MD）快照平均化，以说明溶剂效应和分子柔性。质子化，两性离子和去质子化二肽形式以及天然和d _2-和d _8的多次实验同位素标记的二肽可用于验证结果并评估整体准确性。尽管精度仍然受到实验噪声和计算误差的限制，但已观察到的光谱形状与理论光谱形状非常接近。这样就可以定量测定构象异构体种群，典型分散度为10％。光谱学还证明了构象如何依赖于pH。ROA结果比拉曼结果更一致。通常，ROA分析对实验中的伪影（例如不完全的基线减法）更具抵抗力。MD预测的合格率与实验值相当，但不完全一致。这表明琥珀色力场中存在较小缺陷，特别是对于质子化的二肽而言。全面的，