Using Ionic Strength To Modulate Lipid Diffusion In Solid-Supported Lipid Bilayers

Solid-supported lipid bilayers (SLBs) are an ideal model system to study natural cell membranes. They can be used as a medium for protein separation, a target for virus fusion, and as an integral component of biosensors. These applications often require that the bilayer contain zwitterionic (neutral) and negatively charged lipids in an environment of non-zero ionic strength, as is the case with natural cell membranes. In this work the effect of ionic strength on the fluidity of mixed zwitterionic/anionic solid-supported lipid bilayers is investigated using fluorescence recovery after photobleaching (FRAP). Ionic strength of up to 3000 mM was found to have very little impact on the fluidity of purely zwitterionic bilayers. In the case of the mixed zwitterionic/anionic bilayers however, it was found that even small levels of ionic strength significantly reduced the diffusion coefficient of the anionic lipids . These results were supported by light scattering measurements of vesicles that indicate ions have strong interactions with anionic lipids, but not zwitterionic ones. This work provides insight into the behavior of lipids in natural cell membranes and underscores the importance of appropriate fluorescent probe selection in diffusion measurement studies. Additionally, it demonstrates a technique for controlling the fluidity of lipid bilayers by simply adjusting the ionic strength of the buffer surrounding the sample as opposed to altering the composition or substrate of the bilayer. This technique allows one to investigate the effect of diffusion within one specific sample, eliminating t he effect of batch to batch variability in sample preparation