Dynamics of Sedimentation and Deformation of GUVs Under Different Tonicity Conditions

Abstract

POPC Giant unilamellar vesicles (GUVs) with 1 mol% DiIC18(3) as a fluorescent marker were prepared by electroformation in sucrose solutions with varying osmolarities. These vesicles were resuspended in glucose solutions of different concentrations generating isotonic and hypertonic conditions. Vesicles sediment due to the density difference between the solutions. The movement of the vesicles as they approach the glass surface is studied using SPIM microscopy. We find that the velocity of the GUVs remains constant, given by Stokes law as expected when the distance from the surface is several radii in length. Velocity decreases exponentially as vesicles reach the surface. Vesicle deformation due to interactions with the surface was measured for different osmotic conditions using confocal and SPIM microscopy. Boundary element simulations were performed to model vesicle deformation during sedimentation within a viscous fluid. For isotonic conditions, vesicles are assumed to begin with zero tension and tension is generated through contact with the surface. For the hypertonic case, the same is true but with an initial excess area available. Computationally, the mechanical behavior of the lipid bilayer is simulated using a model that considers two modes of deformation responsible for increases in area strain. The first is the smoothing of suboptical thermal undulations and the second is the direct stretching of the area per lipid molecule. Properties of the lipid bilayer are controlled by adjusting bending and area compressibility moduli. A force field is implemented that takes into account local tension, local curvature force, and gravitational pull. Vesicle sedimentation, deformation, and membrane tension were evaluated as a function of g0, a dimensionless factor relating gravitational and curvature energies. Simulations are in agreement with the experimental results and provide additional information of the deformation of vesicles and sedimentation dynamics.