Dextran diffusion

Therefore, while the independence of dextran diffusion on molecular weight is approximately established at high concentrations, the applicability of Eq. (15) in expressing Dcoop directly as a function of C is in doubt in this case since the predictions of the concentration dependence of Dcoop are not upheld with this material. 

Figure 4.17 Reduced diffusion coefficient as a function of molecular size. Hydrodynamic radii were determined from the diffusion coefficient in water according to Equation 4-4. (a) Diffusion of proteins and peptides in mid-cycle human cervical mucus [5], Measurements were performed by FPR (squares) or quantitative imaging of fluorescence profiles (circles), (b) Protein and dextran diffusion through granulation (squares) or tumor tissue (circles) in the rabbit ear [20, 21]. (c) Glucose diffusion through capsular tissue (squares) and cartilage (circles and triangles) [92]. Each symbol represents a separate measurement.

Figure 4.27 Diffusion coefficients for ficoll in the cytoplasm. Relative diffusion coefficient (T A,cyto/f A,water) f r ficoll diffusion in the cytoplasm of cultured fibroblasts (triangles), albumin diffusion in neurite cytoplasm (circles), and dextran diffusion in neurite cytoplasm (squares). Data from [9, 29].

De Smedt, etal. used FRAP to examine diffusion of fluorescein-labeled dextrans and polystyrene latex spheres through hyaluronic acid solutions(17). Dextrans had molecular weights 71, 148, and 487 kDa. The hyaluronic acid had and of 390 and 680 kDa. The dextran diffusion coefficients depend on matrix polymer c as stretched exponentials in c, as seen in Figure 9.9b. Hyaluronic acid solutions are somewhat more effective at retarding the larger dextran probes. Viscosities for these solutions were reported by De Smedt, et a/. (18). The concentration dependence of rj is stronger than the concentration dependence of Dp of the polystyrene spheres, which is in turn stronger than the concentration dependence of Dp of the dextrans. Spheres and dextrans both diffuse more rapidly than expected from the solution viscosity and the Stokes-Einstein equation. 

M. P. Bohrer, G. D. Paterson, P. J. Carrol. Hindered diffusion of dextran and ficoll in microporous membranes. Macromolecules 77 1170-1173, 1984. 

Elimination from the vitreous occurs by one of two pathways. This can be visualized by injecting fluorescent compounds and examining the concentration distribution in frozen sections obtained after a steady state has been established [230]. If the major route of elimination is by means of the re-tina/choroid, at steady state the lowest concentration would be in the vicinity of the retina. The contours observed in frozen sections of the rabbit eye obtained after intravitreal injection of fluorescein exhibit this pattern, with the highest concentration immediately behind the lens (Fig. 16A). Compounds not chiefly eliminated through the retina exit the vitreous by passive diffusion and enter the posterior aqueous, where they are eliminated by the natural production and outflow of aqueous humor. In such a situation, the contours would be perpendicular to the retina, with the highest concentration towards the rear of the vitreous cavity. This appears to be the case for fluorescently labeled dextran polymer, whose contours decrease in concentration toward the hyaloid membrane (Fig. 16B). 

Passive diffusion is considered to be the major pathway by which xenobiotics cross the placenta. Paracellular diffusion was shown to be the predominant pathway for transfer of hydrophilic solutes, such as chloride ions across perfused placental lobes and opioid peptides and dextrans across BeWo cells [11-13], It has been proposed that denudations in the syncytiotrophoblasts-containing fibrinoid deposits provide a possible paracellular route across the placenta [14], Transtrophoblast channels in the syncytiotrophoblasts could also be responsible for this mode of diffusion [15], For more lipophilic solutes, the transplacental route appears to be the preferred mode of passage... 

A significant size exclusion phenomenon was observed for the IPN membranes. Theophylline R =1.3 A), proxyphylline = 2.3 A), ox-prenolol HCl R = 2.6 A), and FITC-Dextran R = 49 A) were used as model drugs in the diffusion study where R denotes the hydrodynamic radius of the solute. The solute size, membrane mesh size, pH, temperature, and the affinity of the solute with the membrane can affect the permeation of the solute. 

The affinity of the polymer-bound catalyst for water and for organic solvent also depends upon the structure of the polymer backbone. Polystyrene is nonpolar and attracts good organic solvents, but without ionic, polyether, or other polar sites, it is completely inactive for catalysis of nucleophilic reactions. The polar sites are necessary to attract reactive anions. If the polymer is hydrophilic, as a dextran, its surface must be made less polar by functionalization with lipophilic groups to permit catalytic activity for most nucleophilic displacement reactions. The % RS and the chemical nature of the polymer backbone affect the hydrophilic/lipophilic balance. The polymer must be able to attract both the reactive anion and the organic substrate into its matrix to catalyze reactions between the two mutually insoluble species. Most polymer-supported phase transfer catalysts are used under conditions where both intrinsic reactivity and intraparticle diffusion affect the observed rates of reaction. The structural variables in the catalyst which control the hydrophilic/lipophilic balance affect both activity and diffusion, and it is often not possible to distinguish clearly between these rate limiting phenomena by variation of active site structure, polymer backbone structure, or % RS. 

Fig. la-f. The mutual diffusion coefficient (D22)v of dextran as a function of dextran concentration for a dextran T10 (Mw 1(f), b dextran T20 (M 2 x 1(f), c dextran T70 (Mw 7 x 10 ), d dextran FDR7783 (Mw 1.5 x 105), e dextran T500 (Mw 5 x10s), and f dextran T2000 (Mw 2x 106) 0,valuesofD22obtainedbymeasurementofthebyconcentrationgradientrelaxation as monitored by refractive index methods ( ), values of D22 obtained by photon correlation spectroscopy. Data obtained from ref. and unpublished work. For earlier studies of dextran mutual... 

A detailed kinetic analysis of the rapid transport of PVP 360 in the solution of dextran has been made with the aid of a newly developed diffusion cell48) in which horizontal boundaries are formed by a shearing mechanism in a cylinder 5 mm in diameter and 10 mm high (Fig. 4). The transport of radiolabelled [3H]PVP 360 over the boundary does not follow normal diffusional kinetics, i.e. it is not linear with the square root of time (Fig. 5). Instead, the transport appears to be linear with time up to about 3 h, then, it levels off. Within the first 3 h, about 70-80% of the PVP... 

The formation of a boundary between the dextran solution and the dextran solution containing PVP 360 (concentration 5 kg m 3) yields an apparently normal Gaussian distribution of the material detected by Schlieren optics. The various apparent diffusion coefficients obtained by an analysis of the Schlieren curves, which include diffusion coefficients obtained by the reduced height-area ratio method, the reduced second-moment and the width-at-half-height method, show the same qualitative behavior although quantitative differences do exist. This is seen in Fig. 7 where the... 

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