Dextran Stokes radius

FIGURE 2.8 Determination of relative clearance of dextran as a function of solute size. Please note that Stokes radius (left curve) will yield too low a value for the renal clearance barrier. [Reproduced from Hagel et al. (1993), with permission.]... 

Pitkanen et al. [51] reported the isolated bovine RPE-choroid was up to 20 times more permeable to lipophilic than hydrophilic beta-blockers. Furthermore, the in vitro permeability of bovine RPE-choroid to hydrophilic compounds and macromolecules was 10 to 100 times less compared to sclera, whereas the permeability for lipophilic molecules was in the same range for both tissues. The isolated bovine RPE-choroid also exhibited differential permeation by molecular weight and Stokes radius. The permeation rate of 4, 10, and 20 kDa FITC dextrans was moderate compared to a good permeation rate for the 376 Da carboxyfluor-escein and a poor penetration rate for 40 and 80 kDa FITC-dextrans. The permeability to carboxyfluorescein was 35 times more than to 80 kDa FITC-dextran [51]. In a study on the permeability of the human ciliary epithelium to a horseradish peroxidase, Tonjum and Pedersen [52] reported that ciliary and iridial epithelium contained a system of paracellular zonulae occludentes. Peroxidase was applied on the stromal side of ciliary body and iris specimens obtained from freshly enucleated eyes. The 40 kDa peroxidase was blocked apically in the lateral intercellular spaces of the CNPE whereas in the iris the progression of peroxidase was blocked apically in the lateral intercellular spaces of the IPE. Freddo [53] studied the intercellular junctions in the posterior IPE cells of the rhesus monkey by electron microscopy, freeze-fracture, and horseradish peroxidase. Intravenously injected horseradish... 

Fig. 6 Radius of the equivalent hydrodynamic sphere (Stokes radius) for dextran and poly(ethylene oxide) in water. Calculated from the collected data of viscosity and diffusivity. The scales for two lines are displaced from each other for clarity, (Reproduced from ref. 32 with permission.)...

Many polystyrene fractions are available as probe polymer in nonaqueous systems. For aqueous systems, we can use the dextran-oligosaccharide series and poly(ethy1ene oxide) - polyethylene glycol series (polymers that are substantially neutral and with which there will be little possibility of adsorption or conformational change due to the presence of electrolytes). For the latter polymers, the relations between the Stokes radius and molecular weight have been collected as in Fig. 6. 

Fig. 7. Molecular weight dependence of diffusivity. (a) The effective diffusion coefficient, D, has been plotted as a function of molecular weight for dextrans (Nugent and Jain, 1984a, b Gerlowski and Jain, 1986), albumin (Nugent and Jain, 1984a, b), and IgG (Clauss and Jain, 1990) in water, normal tissue, and tumor tissue. Symbols , dextran, aqueous O, bovine serum albumin, aqueous O, rabbit IgG, tumor , dextran, normal tissue , bovine serum albumin normal tissue , rabbit IgG, normal tissue. The half-filled symbols refer to the tumor data, (b) The effective diffusion coefficient plotted versus the Stokes-Einstein radius. Symbols as in (a) plus X, sodium fluorescein, tumor +, sodium fluorescein, normal tissue. (From Clauss and Jain, 1990, with permission.) Currently, we are measuring diffusion coefficient of molecules and particles larger than 50 A in radius.

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