On the models for understanding the second harmonic generation scattering from lipid membranes and the corresponding molecular kinetics

The kinetics of molecular transportation through lipid membranes are key information for understanding many biophysical processes in real cells or protocells. Second harmonic generation (SHG) is a unique surface selective method that may be applied to monitor the adsorption and cross-membrane transport of molecules. Different from many other approaches that probe molecules both in the bulk phase and on the interface, SHG directly probes the kinetics of molecules on interfaces because it exclusively detects molecules/materials with the breaking of inversion symmetry, which normally occurs on interfaces. In this contribution, we summarize some of the literatures on using SHG to probe the dynamic behaviors of molecules on the surface of lipid bilayers including vesicles, bacteria, and cancer cells, mainly focusing on the models that have been used for understanding the SHG signals and revealing the molecular kinetics. The enhancement of the SHG scattering from the adsorption of molecules with chromophores on the membranes and the decay of the SHG signal with a cross-membrane transport are reviewed. The effect of adsorption-desorption equilibrium to the SHG decay during the cross-membrane transport are discussed. Change of the SHG signal with orientational adjustment of the concerned molecules accompanied or not accompanied by their cross-membrane transport are presented. The influence of molecular structures or chemical environment of the surrounding solutions on molecular kinetics at the interfaces are also included. By discussing the previously applied models and showing some examples on how the SHG signals can be analyzed to reveal the kinetic processes, we hope this work may provide valuable information to the researchers in this field, especially the new comers.