Equilibrium partition coefficients (Φ, the concentration in the gel divided by that in free solution) of fluorescein-labeled Ficolls in pure agarose and agarose-dextran composite gels were measured as a function of gel composition and Ficoll size. The four narrow fractions of Ficoll, a spherical polysaccharide, had Stokes-Einstein radii ranging from 2.7 to 5.9 nm. Gels with agarose volume fractions of 0.040 and 0.080 were studied, with dextran volume fractions (calculated as if the chain were a long fiber) up to 0.011. As expected, Φ generally decreased as the Ficoll size increased (for a given gel composition) or as the amount of dextran incorporated into the gel increased (for a given agarose concentration and Ficoll size). The decrease in Φ that accompanied dextran addition was predicted well by an excluded volume theory in which agarose and dextran were both treated as rigid, straight, randomly positioned and oriented fibers. Modeling dextran as a spherical coil within a fibrous agarose gel produced much less accurate predictions. The diffusional permeabilities of these gels were assessed by combining the current partitioning data with relative diffusivities (Kd, the diffusivity in the gel divided by that in free solution) reported previously. The values of ΦK d for a synthetic gel with 8.0% agarose and 1.1% dextran (by volume) were found to be very similar to those for the glomerular basement membrane, a physiologically important material which also has a total solids content of ∼10%.
Bibliographical noteFunding Information:
This work was supported by Grant DK 20368 from the National Institutes of Health. Mr. Kenneth Wright provided essential assistance in the electron beam irradiation of the gels.
Copyright 2008 Elsevier B.V., All rights reserved.
- Agarose gel
- Fiber matrix model
- Steric exclusion