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W R Barger

Publications and source records attributed to W R Barger.

4 recordsLinked to original sources
PubMed2000

Atomic force microscope image contrast mechanisms on supported lipid bilayers.

This work presents a methodology to measure and quantitatively interpret force curves on supported lipid bilayers in water. We then use this method to correlate topographic imaging contrast in atomic force microscopy (AFM) images of phase-separated Langmuir-Blodgett bilayers with imaging load. Force curves collected on pure monolayers of both distearoylphosphatidylethanolamine (DSPE) and monogalactosylethanolamine (MGDG) and dioleoylethanolamine (DOPE) deposited at similar surface pressures onto a monolayer of DSPE show an abrupt breakthrough event at a repeatable, material-dependent force. The breakthrough force for DSPE and MGDG is sizable, whereas the breakthrough force for DOPE is too small to measure accurately. Contact-mode AFM images on 1:1 mixed monolayers of DSPE/DOPE and MGDG/DOPE have a high topographic contrast at loads between the breakthrough force of each phase, and a low topographic contrast at loads above the breakthrough force of both phases. Frictional contrast is inverted and magnified at loads above the breakthrough force of both phases. These results emphasize the important role that surface forces and mechanics can play in imaging multicomponent biomembranes with AFM.

Diglycerides↗
PubMed1998

Characterization of the physical properties of model biomembranes at the nanometer scale with the atomic force microscope.

Interaction forces and topography of mixed phospholipid-glycolipid bilayers were investigated by atomic force microscopy (AFM) in aqueous conditions with probes functionalized with self-assembled monolayers terminating in hydroxy groups. Short-range repulsive forces were measured between the hydroxy-terminated probe and the surface of the two-dimensional (2-D) solid-like domains of distearoyl-phosphatidylethanolamine (DSPE) and digalactosyldiglyceride (DGDG). The form and range of the short-range repulsive force indicated that repulsive hydration/steric forces dominate the interaction at separation distances of 0.3-1.0 nm after which the probe makes mechanical contact with the bilayers. At loads < 5 nN the bilayer was elastically deformed by the probe, while at higher loads plastic deformation of the bilayer was observed. Surprisingly, a short-range repulsive force was not observed at the surface of the 2-D liquid-like dioleoylphosphatidylethanolamine (DOPE) film, despite the identical head groups of DOPE and DSPE. This provides direct evidence for the influence of the structure and mechanical properties of lipid bilayers on their interaction forces, an effect which may be a major importance in the control of biological processes such as cell adhesion and membrane fusion. The step height measured between lipid domains in the AFM topographic images was larger than could be accounted for by the thickness and mechanical properties of the molecules. A direct correlation was observed between the repulsive force range over the lipid domains and the topographic contrast, which provides direct insight into the fundamental mechanisms of AFM imaging in aqueous solutions. This study demonstrates that chemically modified AFM probes can be used in combination with patterned lipid bilayers as a novel and powerful approach to characterize the nanometer scale chemical and physical properties of heterogeneous biosurfaces such as cell membranes.

Diglycerides↗
PubMedDate not supplied

A chemical comparison of methanesulfonyl fluoride with organofluorophosphorus ester anticholinesterase compounds.

The chemical reactivity and physical properties of methanesulfonyl fluoride (MSF) were quantitatively measured for direct comparison with those of isopropyl methylphosphonofluoridate (GB, sarin) and diisopropyl phosphorofluoridate (DFP). The chemistry involved reaction rate constant measurements of MSF hydrolysis and for reactions with phenolic, amine, oxime, hydroxamic acid, phenyl N-hydroxycarbamate, and hydroxylamine compounds and cupric imidazole and bipyridyl complexes. Analogous chemistry with GB and DFP occurred only for the nucleophilic reactions. MSF was substantially less reactive toward interactions involving hydrogen bonding, cupric complex coordination, and quaternary amine formation. Physical characterization included water solubility, vapor pressure, heat of vaporization, solubility parameter, hydrogen bond basicity, UV, IR, and NMR measurements. MSF is a much weaker hydrogen bond acceptor than the phosphorus esters. The hydrogen bond basicity beta-scale measurement provides a key insight to the chemical reactivity divergence of MSF and DFP or GB. Its unexpectedly low value is interpreted that MSF does not possess a site with sufficient basicity to be protonated by strong acid, to enter into hydrogen bonding, or for coordination with the cupric ion.

Cholinesterase Inhibitors↗