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Mechanical Property Measurements of Membranes and Viruses by Using Fluorescence Interference Contrast Microscopy and Atomic Force Microscopy


The elasticities of freestanding curved lipid bilayer arrays formed on micron-diameter wells fabricated on a Si wafer were studied by measuring the height profile using fluorescence interference contrast microscopy. Dark and bright rings resulted from the interference of lipid fluorophore excitation and emission between the direct light and that reflected from the bottom surface of the well. By changing the osmotic pressure difference across the bilayers, the relationship between the pressure and membrane curvature was measured. Using Helfrich theory, the effective surface tension of the bilayer was extracted. Saturated and unsaturated lipids and the influence of cholesterol and ergosterol were investigated. The results show ergosterol decreases the effective surface tension, whereas cholesterol makes the membrane more rigid. The demonstrated technique should find wide application in studying the modification of the elasticities of membranes due to changes in lipid composition or interaction with non-membrane components. The elasticity of membranes was also studied by indenting immobilized intact giant vesicles on glass using an atomic force microscope (AFM) in a fluid environment. Here the buckling force on the vesicles under different force loading rates was used to calculate the Young's modulus E of the lipid membranes. We also imaged brome mosaic virus (BMV) and Sindbis virus (SINV) in a fluid environment by using AFM. Then effective spring constants were measured for both viruses from force indentation experiments. BMV showed a linear response regime for indentation up to 32 % of the diameter. SINV while initially had a nonlinear response which turned into linear for larger forces. Then E for both viruses was calculated by using thin shell model. BMV has an E of 202 MPa which is much closer to that of CCMV where it was 110 MPa for SINV. Finally, the interaction between melittin membranes composed of saturated and unsaturated lipids using the AFM and fluorescence microscopy was studied. We observed that melittin only interacted with unsaturated lipid membrane. It damaged it and transformed the unsaturated lipid membrane into a new thinner bilayer. The role of cholesterol was also studied and found to have a strong role in the interaction.

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