Hindered convection of proteins in agarose gels

The hindered convection of macromolecules in gels was examined by measuring the sieving coefficient (Theta, the ratio of filtrate to retentate concent ration) of globular proteins in agarose membranes, as a function of protein size and gel concentration. The proteins used were lactalbumin (Stokes-Ein stein radius, r(s) = 2.1 nm), ovalbumin (r(s) = 3.0 nm), and BSA (r(s) = 3. 6 nm), and the volume fraction of agarose (phi) was varied from 0.04 to 0.0 8. Agarose membranes were prepared on polyester mesh supports and studied i n a stirred ultrafiltration cell. The Darcy permeabilities of the gels were determined in addition to Theta, and separate measurements of the mass tra nsfer coefficient in the stirred cell were made to correct Theta for the ef fect of concentration polarization. The values of Theta decreased with incr easing r(s) or phi as expected. From the measurements of Theta and estimate s of the protein diffusivity and equilibrium partition coefficient,the conv ective hindrance factor (K-c) was calculated for each protein-gel combinati on. This is the ratio of the average solute velocity (in the absence of dif fusion) to the superficial fluid velocity. For small r(s) and phi it was fo und that K-c slightly exceeded unity, whereas K-c < 1 at larger values of r (s) or phi. This behavior was qualitatively, but not quantitatively, consis tent with predictions from existing hindered transport theories for media c onsisting of parallel fibers or straight pores. Given evidence from previou s partitioning and diffusion data that an agarose gel is better represented as a randomly oriented array of fibers, the large quantitative discrepanci es between the data and models are not entirely surprising. Thus, the prese nt results suggest that there is a need to extend theories of hindered conv ection to random arrays of fibers. (C) 1999 Elsevier Science B.V. All right s reserved.