Poopari Mohammad Reza, Dezhahang Zahra, Xu Yunjie
Department of Chemistry, University of Alberta, Edmonton, Alberta T6G 2G2, Canada.
Department of Chemistry, University of Alberta, Edmonton, Alberta T6G 2G2, Canada.
Spectrochim Acta A Mol Biomol Spectrosc. 2015 Feb 5;136 Pt A:131-40. doi: 10.1016/j.saa.2013.08.118. Epub 2013 Sep 7.
Infrared (IR) and vibrational circular dichroism (VCD) spectra of N-Acetyl-L-Cysteine Methyl Ester (NALCME) and N-Acetyl-L-Cysteine (NALC) in D2O under different pHs were measured. We focus on the VCD signatures of the amide I and the C=O stretching spectral signatures of the neutral NALCME and NALC species and the related ones of the deprotonated NALC species in the region of 1800-1500 cm(-1). A sign inversion is observed for the amide I VCD band going from the neutral NALCME and NALC to the deprotonated NALC species. Density functional theory (DFT) calculations were carried out to search for the possible conformations of these three species and to simulate their IR and VCD spectra at the B3LYP/aug-cc-pVTZ level in the gas phase and with the polarization continuum model of water solvent. The most stable conformations found for neutral NALCME and NALC exhibit drastically difference VCD patterns, whereas those of deprotonated NALC show similar patterns. We establish an empirical structural-spectral relationship where the aforementioned VCD signatures can be used as spectral markers to identify dominant conformations of these two amino acid derivatives under different pHs. It is recognized that the dominant conformers identified using the VCD spectral markers differ from those based on the relative DFT energies for neutral NALCME and NALC. The influence of solvent on both the conformational geometries and their relative stabilities is discussed. The aforementioned discrepancy can be attributed to the explicit solute-solvent hydrogen-bonding interactions which are not accounted for in the calculations. The empirical structural-spectral relationship identified can potentially be applied to large, related amino acids and polypeptides in water.
测量了在不同pH值下，重水（D2O）中N-乙酰-L-半胱氨酸甲酯（NALCME）和N-乙酰-L-半胱氨酸（NALC）的红外（IR）光谱和振动圆二色性（VCD）光谱。我们重点研究了在1800 - 1500 cm⁻¹区域内，中性NALCME和NALC物种的酰胺I的VCD特征以及C = O伸缩光谱特征，以及去质子化NALC物种的相关特征。从中性NALCME和NALC到去质子化NALC物种，观察到酰胺I的VCD带出现符号反转。进行了密度泛函理论（DFT）计算，以寻找这三种物种的可能构象，并在气相中以及使用水溶剂的极化连续介质模型，在B3LYP/aug-cc-pVTZ水平下模拟它们的IR和VCD光谱。中性NALCME和NALC最稳定的构象表现出截然不同的VCD模式，而去质子化NALC的构象则显示出相似的模式。我们建立了一种经验性的结构 - 光谱关系，其中上述VCD特征可作为光谱标记，用于识别这两种氨基酸衍生物在不同pH值下的主要构象。人们认识到，使用VCD光谱标记识别的主要构象与基于中性NALCME和NALC的相对DFT能量识别的构象不同。讨论了溶剂对构象几何形状及其相对稳定性的影响。上述差异可归因于明确的溶质 - 溶剂氢键相互作用，而这些相互作用在计算中未被考虑。所确定的经验性结构 - 光谱关系有可能应用于水中的大型相关氨基酸和多肽。
