(600f) pH-Dependent Formation of Lipid Heterogeneities Controls Surface Topography and Binding Reactivity in Functionalized Bilayers

During direct cell-to-cell communication, lipids on the extracellular side of plasma membranes reorganize, and membrane associated communication-related molecules co-localize. At co-localization sites, sometimes identified as rafts, the local cell surface topography and reactivity are altered. The processes regulating these changes are largely unknown. On model lipid membranes, study of simplified processes that control surface topography and reactivity may potentially contribute to the understanding and control of related cell functions and associated diseases. Integration of these processes on nanometer-sized lipid vesicles used as drug delivery carriers would precisely control their interactions with diseased cells minimizing toxicities.

Here we design such basic pH-dependent processes on model functionalized lipid bilayers, and we demonstrate reversible sharp changes in binding reactivity within a narrow pH window. Cholesterol enables tuning of the membrane reorganization to occur at pH values not necessarily close to the reported pKa's of the constituent titratable lipids, and bilayer reorganization over repeated cycles of induced pH changes exhibits hysteresis.

In the present study, we describe our effort to use the lipid bilayer membrane as the structural foundation whose lateral reorganization into lipid heterogeneities affects the lateral localization of reactive molecules that are attached to it, with implications in the effective reactivity of the membrane ¹. We use information from the current understanding of molecular interactions that drive lipid phase separation in model membranes,^2-5and an external stimulus such as pH⁶ to reorganize a model, functionalized lipid bilayer membrane into lipid heterogeneities. In particular, using pH as a trigger, we demonstrate the formation of lipid heterogeneities occurring in complex bilayer membranes, in the form of vesicles containing (i) a ?domain? forming lipid with titratable anionic headgroups (lipid A,DSPS), (ii) a lipid A with grafted polymer chains (DSPE-PEG), (iii) non-ionizable lipid B with hydrocarbon tails shorter than those in lipid A (DPPC), and (iv) a lipid with grafted functional groups and hydrocarbon tails identical to lipid B (DPPE-biotin). Lowering the pH creates lipid phase separation on the membrane. At high pH values, the lipid A headgroups are charged and repulsion between the headgroups makes the lipid energetically less likely to crystallize. The membrane appears spatially less heterogeneous, and the functional groups are obstructed by surrounding polymer chains. As the pH value is lowered, the anionic A headgroups become protonated, reducing electrostatic repulsion while possibly increasing hydrogen bonding between newly protonated A headgroups. These conditions favor phase separation in which the polymer-conjugated lipids potentially partition into the newly protonated lipid heterogeneities, driven by the dispersive attractive forces between hydrocarbon tails of the same length. In contrast, the functionalized lipids, with shorter hydrocarbon chains, preferentially partition in the areas of lipids with shorter hydrocarbon chains that are depleted in polymer lipids. Therefore, functionalized lipids become exposed and available to interact with their targets increasing the effective binding reactivity of membranes.

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