Electrostatic Effects on Model Membrane Compressibility and Vesicle Adhesion

Abstract

Though vesicles are recognized as effective drug carriers and functionalized with polymers, targeting ligands, imaging agents, and antibodies for drug delivery applications, there is minimal knowledge of the role of lipid bilayer properties in a vesicle's ability to interact with target membranes. Phospholipids within the cellular lipid membrane are chemically diverse, with tail lengths, saturation, and headgroup chemistry varying depending on tissue function and health condition. These function and status dependent compositional observations raise questions on how the underlying membrane properties, such as thickness, fluidity, and compressibility, alter biophysical processes and regulate the transport of vesicles across the membrane. In this work we use compositionally controlled, free-standing large area model biomembranes (LAMB) as an artificial platform to investigate the ramifications of altered phospholipid composition on vesicle-membrane interactions. First, simultaneous area and capacitance measurements of the LAMB allow us to determine the thickness and Young's modulus (compressibility) of a given membrane of varied compositions. In particular, we focus on the effect of headgroup charge, which shows that increasing anionic phospholipid composition increases the membrane stiffness. Next, small (100nm) vesicles composed of DOPC, DOPE, and DOPE-Rh are introduced and subsequently electrostatically adhered to the LAMB. We use Ca2+ ions as a trigger to initiate fusion between the adhered vesicles and LAMB while monitoring fluorescence and capacitance. We observe interesting morphological changes at the membrane interface such as vesicle-vesicle fusion, formation of worm-like vesicle aggregates, and vesicle-membrane fusion. Current work is focused analyzing these events and calculating the activation energy of membrane fusion by simultaneous fluorescence and capacitance measurements. Characterizing the relationship between energy of fusion and vesicle/membrane composition will provide a basis for the rational design of vesicles for targeted membrane interaction.