Atomic force microscope measurements of long-range forces near lipid-coated surfaces in electrolytes

Abstract

The interaction of DMPC (L-alpha-dimyristoyl-1,2-diterradecanoyl-sn-glycero-3-phosphoch oli ne, C36H72NO8P) lipid-coated Si3N4 surfaces immersed in an electrolyte was investigated with an atomic force microscope. A long-range interaction was observed, even when the Si3N4 surfaces were covered with nominally neutral lipid layers. The interaction was attributed to Coulomb interactions of charges located at the lipid surface. The experimental force curves were compared with solutions for the linearized as well as with exact solutions of the Poisson-Boltzmann equation. The comparison suggested that in 0.5 mM KCl electrolyte the DMPC lipids carried about one unit of charge per 100 lipid molecules. The presence of this surface charge made it impossible to observe an effective charge density recently predicted for dipole layers near a dielectric when immersed in an electrolyte. A discrepancy between the theoretical results and the data at short separations was interpreted in terms of a decrease in the surface charge with separation distance.

Thickness and Elasticity of Gram-Negative Murein Sacculi Measured by Atomic Force Microscopy

“…(ii) Elasticity measurements. The elastic properties of the sacculi were investigated by using the technique described by Xu et al (45,46). The sacculi were deposited on a flat Si 3 N 4 surface into which a series of parallel grooves 150 to 400 nm wide and 300 nm deep had been etched.…”

“…Elasticity measurements. Because E. coli sacculi were typically longer than P. aeruginosa sacculi when grown in broth or on solid medium, they were more easily adapted to measurements by using our grooved silicon wafers for elasticity measurements (45). Unfortunately, sacculi from P. aeruginosa were smaller and more fragile than those of E. coli, and elasticity measurements on only a few sacculus bridges could be obtained.…”

“…It can resolve nanometer detail in biological specimens, it can be used underwater so that samples remain hydrated, and it is a topographic imaging technique that can provide exact z axis measurement (e.g., thickness measurements). Because the tip of the microscope is in contact with the specimen and because the force constant of the tip on the specimen is adjustable, AFM can also be used to measure elasticity and rigidity properties of microbial surfaces and long-range forces over membranes (45,46). This is done by determining the Young's modulus of the material in question.…”

The Effect of Lipid Demixing on the Electrostatic Interaction of Planar Membranes across a Salt Solution

“…Again, lipid demixing has turned out to be crucial for the understanding of the underlying mechanism (Chen and Nelson, 2000). It should be noted that similar lipid reorganization phenomena may occur close to the tips of atomic force microscopes close to charge surfaces (Butt, 1991;Xu et al, 1997;Müller et al, 1999). Another recent experiment that must be mentioned in this context is that of Nardi et al (1998Nardi et al ( , 1997 who have studied the adhesion process between a cationic vesicle and an anionic membrane, both simple binary mixtures of neutral and charged lipids serving as model systems for cationic gene delivery vectors.…”

Measurement of the Physical and Chemical Properties of Hybrid Lipid Membranes with the Atomic Force Microscope

“…Shortrange steric/hydration forces and plastic deformation have been measured with the AFM on lipid layers (16,17,18,19). Electrostatic forces on supported PC and PG layers have also been investigated with the AFM by coating both the surface and probe with ho mogeneous lipid layers (20,21). Nanometer resolution relative charge density maps have also been measured on lipid bilayers (22).…”