(408b) pH-Triggered Self-Assembly On Giant Unilamellar Lipid Vesicles

There is growing evidence that nature uses lipid membranes as a universal wrap around cells to control critical functions by dynamically reorganizing lipids into rafts. These are nanometer- to micron-sized lipid domains of laterally phase separated lipids occurring at the sites of action during cell-to-cell communication. Understanding the biophysical forces among lipids that play a role in these phenomena, and characterizing the potential changes of collective physical properties of membranes caused by these phenomena may potentially impact the control of related cell functions and associated diseases.

We study lateral lipid phase separation on bilayers composed of: a first lipid type with a zwitterionic phosphatidylcholine (PC) headgroup, a second lipid type with a titratable headgroup such as phosphatidic acid (PA) or phosphatidyl serine (PS), and cholesterol in the form of Giant Unilamellar Vesicles (GUVs). These studies involve systematic changes of temperature and pH affecting the balance between electrostatic, hydrogen bonding, and van der Waals interactions among lipids forming heterogeneous lateral assemblies; the aim is to identify the role of each type of intermolecular interaction on affecting the extent of formation and the shape of phase-separated domains. Measurements include investigation of the morphology, as well as the kinetics of formation and growth of phase separated lipid domains, the potential role of cholesterol and of the ionic strength. In these studies we use fluorescence microscopy to image model heterogeneous lipid bilayers in the form of GUVs.

Our findings suggest a significant role for cholesterol on acting against phase separation of the gel/fluid lipid mixture and of formation of gel phase domains; a role that is possibly obstructed by lowering pH when hydrogen bonding among the titratable lipid headgroups becomes more pronounced. The shape of phase separated domains increasingly resembles florets with lowering pH independently of the path followed; it is suggested that these domain morphologies retain the integrity of the entire giant vesicle.