Hindered convection of macromolecules in hydrogels

Hindered convection of macromolecules in gels was studied by measuring the sieving coefficient (Theta) of narrow fractions of Ficoll (Stokes-Einstein radius, r(s) = 2.7 - 5.9 nm) in agarose and agarose-dextran membranes, along with the Darcy permeability (kappa). To provide a wide range of kappa, varying amounts of dextran (volume fractions less than or equal to 0.011) were covalently attached to agarose gels with volume fractions of 0.040 or 0.080. As expected, Theta decreased with increasing r(s) or with increasing concentrations of either agarose or dextran. For each molecular size, Theta plotted as a function of kappa fell on a single curve for all gel compositions studied. The dependence of Theta on kappa and r(s) was predicted well by a hydrodynamic theory based on flow normal to the axes of equally spaced, parallel fibers. Values of the convective hindrance factor (K-c, the ratio of solute to fluid velocity), calculated from Theta and previous equilibrium partitioning data, were unexpectedly large; although K-c less than or equal to 1.1 in the fiber theory, its apparent value ranged generally from 1.5 to 3. This seemingly anomalous result was explained on the basis of membrane heterogeneity. Convective hindrances in the synthetic gels were quite similar to those in glomerular basement membrane, when compared on the basis of similar solid volume fractions and values of kappa. Overall, the results suggest that convective hindrances can be predicted fairly well from a knowledge of kappa, even in synthetic or biological gels of complex composition.