Quantitative functional group orientation in Langmuir films by infrared reflection-absorption spectroscopy:: C=O groups in behenic acid methyl ester and sn2-13C-DSPC
Quantitative functional group orientation in Langmuir films by infrared reflection-absorption spectroscopy:: C=O groups in behenic acid methyl ester and sn2-13C-DSPC
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DOI：
10.1021/jp030066r
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发表时间：
2003-07-24
影响因子：
3.3
通讯作者：
Mendelsohn, R
中科院分区：
文献类型：
--
作者：
Brauner, JW;Flach, CR;Mendelsohn, R
用红外反射-吸收光谱法（IRRAS）测定了水溶液单层膜中山嵛酸甲酯（BME）和sn 2-C-13 - 1，2-二硬脂酰磷脂酰胆碱（sn 2-C-13-DSPC）中羰基的取向。BME的C=O伸缩轮廓由三个重叠的峰组成，分别在1737、1721和1702 cm（-1）处由未水合、单水合和双水合形式产生。用V.L.的光学理论模拟了由轮廓分辨分量的p-和s-偏振IRRAS强度构造的二向色性比。Kuzmin（1，2）基于价键和分子建模考虑，羰基基本上垂直于链轴。IRRAS确定的过渡时刻确实位于沿着C=O键的未水合基团（从而提供了良好的支持理论），但改变方向的水合基团。通过假设羰基刚性连接的锯齿形全反式亚甲基链的单轴分布，无法获得模拟与实验之间的满意拟合。对于BME和Sn 2-C-13 DSPC，发现链平面在垂直于槽障运动方向上的轻微优先取向。此外，BME在1000-1300 cm（-1）区域的光谱质量足以确定酯部分的单键C-O伸缩的取向在垂直方向的20度内。对于sn 2-C-13-DSPC，每个C=O键拉伸轮廓被分成由水合和未水合形式产生的双峰。对IRRAS数据的最佳拟合为每条链中的未水合C=O模式导致建议的30度的链倾斜。该值与X射线衍射测量结果雅阁，通过分析CH 2伸缩振动证实了3。在sn 2-C-13-DSPC中的水合C=O键的过渡时刻再次被发现不位于沿着键方向。本分析提供了一种新的方法来确定功能团在单分子膜中的取向。该方法，当与X射线和中子反射率测量相结合时，将提供关于膜成分的结构和相互作用的独特信息。
IR reflection-absorption spectroscopy (IRRAS) is used to determine the orientation of carbonyl groups in behenic acid methyl ester (BME) and in sn2-C-13 1,2 distearoylphosphatidylcholine (sn2-C-13-DSPC) in aqueous monolayer films. The C=O stretching contour of BME consists of three overlapped peaks arising from unhydrated, monohydrated, and doubly hydrated forms at 1737, 1721, and 1702 cm(-1), respectively. Dichroic ratios constructed from the p- and s-polarized IRRAS intensities of the resolved components of the contour were simulated with the optical theory of V. L. Kuzmin.(1,2) On the basis of valence bond and molecular modeling considerations, the carbonyl group is essentially perpendicular to the chain axes. The transition moment is determined by IRRAS to indeed lie along the C=O bond for the unhydrated groups (thus providing good support for the theory) but changes direction for hydrated groups. A satisfactory fit between simulation and experiment could not be obtained by assuming a uniaxial distribution of the zigzag all-trans methylene chain to which the carbonyl group is rigidly attached. A slight preferential orientation of the chain planes in the direction perpendicular to that of the trough barrier movement is found for both BME and sn2-C-13 DSPC. In addition, spectral quality in the 1000-1300 cm(-1) region for BME was sufficient to determine that the orientation of the single-bond C-O stretch of the ester moiety was within 20degrees of the vertical. For sn2-C-13-DSPC, each C=O bond stretching contour was split into a doublet arising from hydrated and unhydrated forms. The best fit to the IRRAS data for the unhydrated C=O mode in each chain led to a suggested chain tilt of 30degrees. This value, in good accord with X-ray diffraction measurements, 3 was confirmed through analysis of the CH2 stretching vibrations. The transition moments of the hydrated C=O bonds in sn2-C-13-DSPC were again found to not lie along the bond direction. The current analysis provides a new method for determining functional group orientation in monolayers. The approach, when coupled with X-ray and neutron reflectivity measurements, will provide unique information about the structures and interactions of film constituents.