Feng Rong-Juan, Li Xia, Zhang Zhen, Lu Zhou, Guo Yuan
Beijing National Laboratory for Molecular Sciences and State Key Laboratory of Molecular Reaction Dynamics, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China.
J Chem Phys. 2016 Dec 28;145(24):244707. doi: 10.1063/1.4972564.
The interfacial behavior of the benchmark zwitterionic phospholipid molecule dipalmitoylphosphatidylcholine (DPPC) has been extensively investigated by surface-selective vibrational sum frequency generation spectroscopy (VSFG). However, there is still a lack of agreement between various orientational measurements of phospholipid monolayers at the air/water interface, mainly because of the difficulty in assigning congested VSFG features. In this study, polarization-dependent VSFG measurements reveal a frequency shift between the in-plane and out-of-plane antisymmetric stretching modes of the terminal methyl groups in the DPPC alkyl tails, favoring the model of C local symmetry rather than the previously assumed C symmetry. Further VSFG experiments of isotopically labeled DPPC successfully capture the vibrational signatures of the glycerol backbone. With the newly derived VSFG polarization selection rules for C symmetry and the refreshed spectral assignments, the average tilt angles of the alkyl tail groups, choline headgroup, and glycerol backbone of DPPC molecules can all be determined, showing the powerful capability of VSFG spectroscopy in revealing the structural details at interfaces. The VSFG polarization dependence rules and the orientational analysis procedures developed for C symmetry in this work are applicable to other bulky molecules in which the methyl group cannot freely rotate, and they therefore have general applications in future VSFG studies.
基准两性离子磷脂分子二棕榈酰磷脂酰胆碱（DPPC）的界面行为已通过表面选择性振动和频光谱（VSFG）进行了广泛研究。然而，对于磷脂单层在空气/水界面的各种取向测量结果仍缺乏一致性，主要原因在于难以确定拥挤的VSFG特征。在本研究中，偏振相关的VSFG测量揭示了DPPC烷基链末端甲基的面内和面外反对称伸缩模式之间的频率偏移，这支持了C局部对称性模型而非先前假设的C对称性模型。对同位素标记的DPPC进行的进一步VSFG实验成功捕捉到了甘油主链的振动特征。借助新推导的C对称性VSFG偏振选择规则和更新后的光谱归属，DPPC分子的烷基链基团、胆碱头部基团和甘油主链的平均倾斜角均可确定，这显示了VSFG光谱在揭示界面结构细节方面的强大能力。本工作中为C对称性开发的VSFG偏振依赖性规则和取向分析程序适用于甲基不能自由旋转的其他大分子，因此在未来的VSFG研究中具有广泛应用。
