Investigation of Drug–Model Cell Membrane Interactions Using Sum Frequency Generation Vibrational Spectroscopy: A Case Study of Chlorpromazine

Sum frequency generation (SFG) vibrational spectroscopy and attenuated total reflection Fourier transform infrared (ATR-FTIR) spectroscopy were applied to study interactions between an antipsychotic agent, chlorpromazine (CPZ), and model cell membranes consisting of either distearoylphosphatidylcholine (DSPC) or dipalmitoylphosphatidylglycerol (DPPG). The PC and PG lipids represent the zwitterionic and anionic components of the cell membranes, respectively. For an isotopically asymmetric bilayer composed of a deuterated lipid leaflet and a hydrogenated lipid leaflet, the time-dependent SFG signals from the lipids revealed that CPZ can significantly accelerate the flip-flop process of the neutral DSPC bilayer and such an acceleration effect is more pronounced at higher CPZ concentrations. While for the negatively charged DPPG bilayer, it was found that CPZ molecules can immediately bind to and disrupt the outer lipid leaflet and then gradually reduce the ordering of the inner lipid leaflet. A higher CPZ concentration in the subphase leads to a faster disordering effect on the inner leaflet. The association of CPZ to the lipid membranes can be verified by the change in the SFG spectra of the OH stretching vibration of the interfacial water molecules. ATR-FTIR results revealed that addition of CPZ to the subphase did not exert significant effect on the dDSPC/dDSPC bilayer, especially at low CPZ concentrations (<2 mM). It was found that CPZ can cause gel-to-fluid phase transition of the dDPPG/dDPPG bilayer at CPZ concentrations below 2 mM, and higher CPZ concentrations can lead to dissolution of the bilayer. This work demonstrated that SFG (along with ATR-FTIR) is a powerful in situ and label-free technique that can be used to study various aspects of the drug–membrane interactions at the molecular level.