| The cell membrane system is a complex multi-component system,that contains different types of lipids,sterols,sugars and proteins.Phosphatidylethanolamine（PE）is the second abundant zwitterionic phospholipid molecule in the cell membrane,which is involved in many physiological activities such as material transmission,signal transmission,endocytosis and exocytosis between cell membranes.Its structure and properties are affected by the electrolyte environment around the cell membrane.Therefore,it is very important to reveal the interaction between PE and its surrounding ions at the molecular scale.In order to simplify the model,Langmuir-Blodgett monolayer technology is widely used in the study of simulating cell membranes in vitro.There are many technologies can characterize the lipid interface,such as Fourier transform infrared spectroscopy,Raman spectroscopy,atomic force microscopy,and so on.However,due to the complexity of lipid membrane interfaces,it is difficult to describe the changes of molecular structure and conformation at lipid interfaces by traditional characterization methods.Owing to the merits of interface/surface specificity and sub-monolayer sensitivity,the sum frequency generation（SFG）process can probe the interfacial molecules without the interference from the molecules in the bulk（the media with centrosymmetric structures separated by the interface）.Additionally,sum frequency generation vibration spectroscopy（SFG-VS）,which can collect the SFG signals enhanced by the vibrational modes of molecular groups,has been developed into an ideal tool to characterize the molecular structures of lipid molecules at various types of interfacesIn this paper,the assembly kinetics of PE monolayers at the gas/liquid interface,the interaction between lipids and ions,and the calculation of tilt angles of functional groups were studied by using sum frequency vibrational spectroscopy combined with the surface pressure-area isotherm measurements.First,the surface pressure-area isotherms of DMPE and DPPE monolayer at the air/water interface and the air/different kinds of ion interfaces were measured.It was found that both the type and concentration of ions can cause PE monolayers to expand to different degrees,indicating that the addition of ions makes PE molecules occupy a larger mean molecule area.And the concentration-dependent change of the PE monolayers was found in the presence of ions.Second,the SFG-VS spectra of the DMPE monolayer in different wave number regions at the air/water interface and the air/Mg Cl2of different concentrations interface were measured.It is found that the tilt angles of the methyl in tail groups decrease while the tilt angles of the phosphate and methylene in head groups increase during the compression of the DMPE monolayers at both the air/water interface and the air/Mg Cl2solution interfaces.It is also shown that the tilt angle of the methyl in tail groups slightly decreases,and the tilt angles of the phosphate and methylene in head groups significantly increase as the Mg Cl2concentration increases from 0 M to 1.0 M.These studies reveal the changes in the structure and conformation of the head group and tail group of PE molecule under the influence of metal ions,provide the mechanism of the interaction between the ions and PE molecules at the molecular level,and help us understand many cell membrane life processes such as endocytosis,exocytosis,and information transmission between cells. |
