Interaction of Polyethylenimine with Model Cell Membranes Studied by Linear and Nonlinear Spectroscopic Techniques

Polyethylenimine (PEI) has been widely used as a transfection agent for gene delivery, but it is cytotoxic and can lead to cell apoptosis. Although several apoptotic mechanisms have been proposed, a molecular level understanding of PEI/cell membrane interaction can help develop further insight into such cytotoxicity. We combined sum frequency generation (SFG) vibrational spectroscopy and attenuated total-internal reflection Fourier transform infrared (ATR-FTIR) spectroscopy to study the effect of PEI on lipid transbilayer movement in supported bilayers (serving as model cell membranes) as a function of lipid composition, PEI concentration, and temperature. For both dipalmitoylphosphatidylglycerol (DPPG) and distearoylphosphatidylcholine (DSPC) bilayers, PEI molecules showed no significant effect on lipid translocation at room temperature (21 C). In contrast, significant lipid translocation was observed near the physiological temperature (39 C), indicating the ability of PEI to induce lipid translocation in both negatively charged and zwitterionic lipid bilayers, without the assistance of membrane proteins. Furthermore, results showed that PEI had strong interactions with negatively charged DPPG and weak interactions with zwitterionic DSPC. Concentration-dependent studies indicated that the lipid translocation rate had a linear dependence on the PEI concentration in the subphase. The effects of branched and linear PEls were compared in the study, showing that branched PEI had a greater effect on the lipid translocation rate due to the higher charge density, which might be a possible indication of higher toxicity. ATR-FTIR spectroscopy verified that the results observed in SFG were mainly caused by lipid translocation, not bilayer damage or removal from the substrate. The combined SFG and ATR-FTIR study provides a powerful method to examine molecular interactions between lipid bilayers and polyelectrolytes at a molecular level. The results can help to develop further understanding on PEI's cytotoxicity in biological systems.