The transport of small molecules across a phospholipid membrane is studied by molecular dynamics simulations. The effects of size, hydrophobicity, and asphericity of the penetrants on the permeation process are investigated. For this purpose, permeability coefficients of oxygen and ammonia are computed using an inhomogeneous solubility−diffusion model and compared to the previously computed results of the permeation of water. Furthermore, solubility and diffusion data are computed for a series of Lennard-Jones particles that differ in size and shape. The results are discussed within the framework of the four-region model and are especially related to the free volume characteristics of the membrane. It is concluded that the free energy of solvation mainly determines the shape of the permeation resistance profile. For hydrophobic particles the membrane interior will act as a trap instead of a barrier. Moderately hydrophilic and hydrophilic penetrants experience the largest resistance to permeation in the de...

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Permeation Process of Small Molecules across Lipid Membranes Studied by Molecular Dynamics Simulations

Diffusion of carbon dioxide through lipid bilayer membranes: Effects of carbonic anhydrase, bicarbonate, and unstirred layers

Diffusion of (14)C-labeled CO(2) was measured through lipid bilayer membranes composed of egg lecithin and cholesterol (1:1 mol ratio) dissolved in n-decane. The results indicate that CO(2), but not HCO(3-), crosses the membrane and that different steps in the transport process are rate limiting under different conditions. In one series of experiments we studied one-way fluxes between identical solutions at constant pCO(2) but differing [HCO(3-)] and pH. In the absence of carbonic anhydrase (CA) the diffusion of CO(2) through the aqueous unstirred layers is rate limiting because the uncatalyzed hydration-dehydration of CO(2) is too slow to permit the high [HCO(3-)] to facilitate tracer diffusion through the unstirred layers. Addition of CA (ca. 1 mg/ml) to both bathing solutions causes a 10-100-fold stimulation of the CO(2) flux, which is proportional to [HCO(3-)] over the pH range 7-8. In the presence of CA the hydration- dehydration reaction is so fast that CO(2) transport across the entire system is rate limited by diffusion of HCO(3-) through unstirred layers. However, in the presence of CA when the ratio [HCO(3-) + CO(3=)]:[CO(2)] more than 1,000 (pH 9-10) the CO(2) flux reaches a maximum value. Under these conditions the diffusion of CO(2) through the membrane becomes rate limiting, which allows us to estimate a permeability coefficient of the membrane to CO(2) of 0.35 cm s(-1). In a second series of experiments we studied the effects of CA and buffer concentration on the net flux of CO(2). CA stimulates the net CO(2) flux in well buffered, but no in unbuffered, solutions. The buffer provides a proton source on the upstream side of the membrane and proton sink on the downstream side, thus allowing HCO(3-) to facilitate the net transport of CO(2) through the unstirred layers.

https://rupress.org/jgp/article-pdf/69/6/779/1246409/779.pdf

Diffusion of carbon dioxide through lipid bilayer membranes: effects of carbonic anhydrase, bicarbonate, and unstirred layers.

Cells of the established line NHIK 3025 were incubated with hematoporphyrin derivative and exposed to light at different concentrations of oxygen The efficiency of photoinactivation of the sensitized cells decreased with decreasing oxygen concentration No photoinactivation was observed when the atmosphere above the medium overlying the cells was pure N 2 gas With 1% O 2 in the atmosphere, the quantum yield of photoinactivation was reduced by 50% compared to the yield in airsaturated medium It is hardly possible to inactivate cells in anoxic regions of a tumor by means of porphyrin sensitized photochemotherapy In spite of this, the therapy seems to be efficient in several cases Thus, it seems that anoxic tumor cells are inactivated in secondary reactions, probably due to break-down of the circulatory system in the tumor

https://cancerres.aacrjournals.org/content/canres/45/4/1608.full.pdf

Oxygen dependence of the photosensitizing effect of hematoporphyrin derivative in NHIK 3025 cells.

Stereological analyses of electron micrographs were used to quantify physiologically important ultrastructures of slow-twitch oxidative (red) and fast-twitch glycolytic (white) muscle fibers from striped bass (Morone saxatilis) acclimated to 25 and 5 degrees C. The fraction of cell volume occupied by the mitochondria [volume density, Vv (mit,f)] of red fibers increases from 0.286 +/- 0.018 to 0.448 +/- 0.024 between 25 and 5 degrees C; Vv (mit,f) of white fibers increased from 0.027 +/- 0.003 and 0.040 +/- 0.004 at 25 and 5 degrees C, respectively. Because of a concomitant increase in the mass of oxidative muscle, acclimation from 25 to 5 degrees C results in an increase in total mitochondrial volume per 100 g body wt from 2.58 to 6.73 cm3 in oxidative muscle and from 2.46 to 3.40 cm3 in fast glycolytic muscle. Mitochondria of red fibers are in more clustered arrays after cold acclimation. Size and cristae surface densities of individual mitochondria are not affected markedly by acclimation, suggesting true cold-induced proliferation rather than enlargement of organelles. Harmonic means of intermitochondrial spacing in red fibers decreases from 2.64 to 1.43 micron between 25 and 5 degrees C. This reduces diffusion path lengths between sarcoplasmic and mitochondrial compartments proportionately, compensating for decreases in diffusivity of aqueous solutes. Intracellular lipid droplets of red fibers markedly increase in volume density from 0.006 +/- 0.003 at 25 degrees C to 0.079 +/- 0.014 at 5 degrees C.(ABSTRACT TRUNCATED AT 250 WORDS)

Thermal acclimation induces adaptive changes in subcellular structure of fish skeletal muscle.

The solubilities of He, Ne, Ar, N2, O2, CO and CO2 in olive oil have been determined in the temperature range 24–56 C. The gas solubility apparatus was a considerably improved version of the one reported by Morrison and Billett and gave a precision, depending on the gas solvent system of ±0.2% to ±1.0%. Plots of log L, the Ostwald coefficient, and log α, the Bunsen coefficient, against 1/TK were linear and showed a small temperature dependence for CO2. The enthalpy and entropy changes corresponding to the solution process were calculated. The partition coefficients of the gases between olive oil and water were calculated using reported values for the solubility of the gases in water. These results have been used to test and extend the Meyer-Overton theory of transport through the cell membrane.

Francis David Evans

Papers

The solubilities of seven gases in olive oil with references to theories of transport through the cell membrane

Preparation of very pure hexafluorobenzene (>99.99 percent) by directional freezing