Diffusion compounds

Farmer (6) reviewed the various diffusion models for soil and developed solutions for several of these models. An appropriate model for field studies is a nonsteady state model that assumes that material is mixed into the soil to a depth L and then allowed to diffuse both to the surface and more deeply into the soil. Material diffusing to the surface is immediately removed by diffusion and convection in the air above the soil. The effect of this assumption is to make the concentration of a diffusing compound zero at the soil surface. With these boundary conditions the solution to Equation 8 can be converted to the useful form ... 

StumpfWE. Techniques for the autoradiography of diffusible compounds, in Methods in Cell Biology, Vol. 13, Academic Press, New York, 1976. 

Equilibrium dialysis of homogenates of kidneys of rats given mercuric chloride, revealed that over 99% of the mercury was not diffusible [40]. Diffusible compounds of mercury have the opportunity to cross the capillary membrane and enter the tissue spaces however, due to chemical affinities for cellular binding sites and the diffusible complex, and the ability to penetrate the cell membrane, not all diffusible complexes of mercury present in plasma lead to tissue accumulation. 

Comparison of various techniques of autoradiography for diffusible compounds clearly demonstrates that no solutions can be used in processing the tissue. These investigators have dried thin sections of liver and uterus at temperatures below... 

C. These freeze-dried sections were dry mounted on microscope slides which had been precoated with either Kodak NTB-3 or NTB-10 emulsion. Other techniques which thawed the frozen section, embedded the tissue in paraffin or dipped the section in liquid emulsion were demonstrated to translocate diffusible compounds. Many other similar attempts have been and are currently being made to localize diffusible compounds by autoradiography at the electron microscope level. 

Two observations were made by the authors from their log PS data. First, disregarding the physical properties of compounds, the order of BBB permeability was active uptake compounds > passive diffusion compounds > efflux compounds. The average log PS of the two active uptake substrates was approximately one log unit greater than that of passive diffusion substrates the average of log PS of diffusion substrates was approximately one log unit greater than that of efflux substrates. Second, basic compounds appeared to have higher BBB permeability than neutral and acidic compounds in this data set. The rank order of the average log PS values for the passive diffusion compounds by... 

A stepwise multivariate LR analysis of the log PS values of the 23 diffusion compounds and 50 descriptors yielded a linear equation that consisted of 10 descriptors. After considering the relevance in physical meaning of each descriptor and statistical significance, the 10-descriptor model was reduced to a 3-descriptor model (Eq. 68) ... 

Stumpf WE, Roth LJ. 1966. High resolution autoradiography with dry mounted, freeze-dried frozen sections. Comparative study of six methods using two diffusible compounds H-estradiol and H-mesobilirubinogen. J Histochem Cytochem 14 274-287. 

Finally, the diffusion of a chemical may be influenced by another diffusing compound or by the solvent. The latter effect is known as solute-solvent interaction it may become important when solute and solvent form an association that diffuses intact (e.g., by hydration). This may be less relevant for neutral organic compounds, but it plays a central role for diffusing ions. But even for noncharged particles the diffusivities of different chemicals may be coupled. The above example of the glycerol diffusing in water makes this evident in order to keep the volume constant, the diffusive fluxes of water and glycerol must be coupled. 

Until now, we have tacitly assumed that the diffusing compound does not interact chemically with the solid matrix of the porous media. Yet, in a porous medium the solid-to-fluid ratio is several orders of magnitude larger than in the open water. 

In another system, a non-diffusible compound reacts with undeveloped silver ions to liberate a diffusible dye, producing a positive transferred image.141 Again, thiosulfate-containing developers are used. In this system, the silver ions catalyze the ring-opening hydrolysis of a thiazolidine (73), shown in equation (1). 

FIGURE 9.5 Graph showing initial velocity of transport processes across lipid membranes. Passive diffusion (compound dissolves directly into lipid membrane) is driven by a concentration gradient and is not saturable. In contrast, carrier-mediated transport is saturable, reaching a maximal rate when the carrier molecules are saturated with substrate. Transport proteins mediate these processes. 

Masungi C, Borremans C, Willems B, Mensch J, Van Dijck A, Augustijns P, Brewster M, and Noppe M. Usefulness of a Novel CACO-2 Cell Perfusion System I. In Vitro Prediction of the Absorption Potential of Passively Diffused Compounds./Pfearro Set 2004 93 2507-2521. 

The physical properties and chemistry of NO determine its biological fate and potential sites for interaction. NO is a highly diffusible compound because of its low molecular weight and reasonable hydrophobicity and solubility, NO is theoretically able to reach anywhere within cells and tissues (Feldman et al., 1993). Practical limits exist on the diffusion potential of NO, however, due to its many possible chemical interactions. The greatest factor limiting the availability of NO may be its rapid interaction with... 

The parallel artificial membrane permeability assay (PAMPA) is a recent development in the area of artificial membranes that appears to offer considerable potential. Measuring the flux values (membrane permeation levels) of a range of test compounds by PAMPA and relating these values to the flux curves obtained in Caco-2 studies have shown good correlations, indicating that the PAMPA assay could be a good alternative to Caco-2 cells for the measurement of passively diffusing compounds. 

A further complication is the recent discovery that gray and white matter have different radioabsorbencies for tritium (Alexander et al., 1981). An approach to this problem soaks adjacent sections in a freely diffusable compound such as H-isoleudne to permit appropriate compensation for differential absorbencies by different areas (Taylor et al., 1984). Further discussions of this and other issues are presented by Rogers (1979), Kuhar (1982, 1983, 1984), Wamsley and Palacios (1983), and Young and Kuhar (1979a, 1981). 

The permeability of shells is defined by many factors, such as its structure, thickness of shells, a charge of diffused compounds, conditions of microenvironment, a core material. Change of polyelectrolyte shell permeability is possible by variation of microenvironment parameters such as pH, polarity of solvent, an ionic force, temperature. 

So far as natural inductors are concerned, their study has been handicapped for several reasons, mainly the negligible concentrations of active diffusible compounds and the difficulty in obtaining adequate quantities of embryonic tissues. 

Various metal borides can be prepared by molten salt electrolysis. Frazer et al. (1975) have deposited ZrB on nickel cathode from ZrO and B Oj dissolved in molten NajAlF at 1020°C. A graphite crucible was used as anode. A series of nickel boride diffusion compounds were also noticed by electron microprobe analysis. The ZrB deposit was found to be scaly or dendritic and was usually non-adherent. Devyatkin (2001) has also obtained ZrB on nickel cathode from cryolite- alumina melts containing zirconium and boron oxide. 

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