William Z. Plachy

The diffusion-solubility of oxygen in lipid bilayers.

The product, Do alpha, of the oxygen diffusion coefficient, Do, and the oxygen solubility, alpha, is determined in phosphatidylcholine bilayers at temperatures above the lipid phase transitions from ESR spin-exchange measurements. The resulting values of Do alpha are in good agreement with those obtained from fluorescence-quenching experiments. The use of fatty acid spin labels makes it possible to measure Do alpha as a function of the coordinate perpendicular to the bilayer surface. The results indicate that do alpha is a strong function of this coordinate; it is greatest in the bilayer center and least near the bilayer head groups.

Dioxygen solubility in aqueous phosphatidylcholine dispersions.

The solubility of molecular oxygen, or dioxygen, in low weight percent (1.5%) sonicated dimyristoylphosphatidylcholine (DMPC) aqueous dispersions saturated with air has been measured as a function of temperature between 10 degrees C and 40 degrees C. A modified Winkler technique was used involving a dual cell coulometric titration with voltammetric endpoint detection in a mixed solvent (methanol/water). The results indicate that dioxygen is approximately four times more soluble in the liquid crystalline bilayers (above 24 degrees C) than in the gel state bilayers (below 24 degrees C). The solubility of dioxygen in the bilayer does not appear to be strongly temperature dependent on either side of the 24 degrees C phase transition. The dioxygen solubility in gel state DMPC is approximately equal to that in water at the same temperature. Our result are contrasted with recent measurements made using EPR spin labels.

Partitioning of hydrophobic probes into lipopolysaccharide bilayers

Lipophilic solutes permeate rapidly through lipid bilayer membranes. However, the outer membrane of enteric bacteria, which is composed of a lipopolysaccharide monolayer outer leaflet and the glycerophospholipid inner leaflet, shows extremely low permeability to hydrophobic solutes. In order to examine the cause of this exceptionally low permeability, the lipid/water partition behavior of various lipophilic probes was determined by using lipopolysaccharides of various chemotypes and glycerophospholipids. With all probes, under many different conditions, the lipopolysaccharide/water partition coefficients were generally about an order of magnitude smaller than the phospholipid/water partition coefficients, and this result is consistent with the low permeability of the lipopolysaccharide monolayer, and hence the asymmetric bilayer found in the outer membrane. Furthermore, organic polycations significantly increased the partition of N-phenylnaphthylamine into lipopolysaccharides, a result again consistent with the permeability-increasing effect of such cations on intact outer membrane. Very defective, deep rough lipopolysaccharides of chemotypes Rd2, Rd1 and Re, had only slightly (20-75%) higher partition coefficients in comparison with the more complete lipopolysaccharides, and this difference is probably not enough to explain the approximately 100-fold increase in lipophile permeability seen in deep rough strains.

A gas-permeable ESR sample tube

Abstract A technique is described by which the oxygen concentration in a liquid ESR sample can be conveniently controlled while the sample remains in the microwave cavity. A thin-wall polytetrafluoroethylene sample tube, which is permeable to oxygen and nitrogen but not to most liquids, allows the oxygen concentration in the sample to come to equilibrium with the oxygen concentration in the gas stream which is used to maintain temperature control. Thus, by regulating the concentration of oxygen in the gas stream, the partial pressure of oxygen in the sample can be varied from 0 to 1 atm.

Dioxygen diffusion in the stratum corneum: an EPR spin label study

The stratum corneum, the outer 10 microns of the skin, serves as a permeability barrier regulating the transport of molecules between the body and the environment. The purpose of this study is to understand this permeability barrier function as it pertains to the diffusion of molecular oxygen. The stratum corneum was investigated with EPR spectroscopy following inoculation with a stearic acid spin probe. The presence of paramagnetic molecular oxygen results in the broadening of the EPR spectral lines of the spin probe. The rate of oxygen diffusion across the stratum corneum, and then the oxygen diffusion coefficient, D(O2), was determined by studying this line-broadening as a function of time. D(O2) in human stratum corneum was found to be 3 x 10(-7) cm2/s at 37 degrees C with an activation energy of approx. 44 kJ/mol. The application of the permeation-enhancing chemicals, DeMSO and DMSO, to the stratum corneum increased D(O2) two- to three-fold.

X-Ray Diffraction and Electron Paramagnetic Resonance Spectroscopy of Mammalian Stratum Corneum Lipid Domains

Publisher Summary This chapter discusses the application of X-ray diffraction and electron paramagnetic resonance (EPR) spectroscopy to elucidate the structure and organization of the lipid-enriched cellular peripheral domains of the stratum corneum. The two techniques provide confirmatory and complementary information about structure and physical properties on a molecular level. Traditionally, differential scanning calorimetry (DSC) is widely used to obtain information about the heat-induced phase changes in biological membranes, but it must be emphasized that calorimetry does not probe membrane structure. EPR has provided information about the stratum corneum that is both parallel and complementary to information provided by X-ray diffraction. Both techniques provide information on a molecular level about temperature-dependent phase behavior that correlates with DSC determinations. However, X-ray diffraction provides more direct information about structure, whereas EPR provides more direct information about the physical properties of membranes, including polarity, microviscosity, and phase transitions.