Depletion theory of protein transport in semi-dilute polymer solutions

We consider the effect of polymer depletion on the transport (diffusion and electrophoresis) of small proteins through semi-dilute solutions of a flex ible polymer. A self-consistent field theory may be set up in the important case of quasi-ideal interactions when the protein is small enough. Dynamic depletion, the reorganization of the depletion layer as the protein diffus es, is computed within a free-draining approximation. The transport of the dressed particle (protein + depletion layer) is tackled by extending Ogston 's analysis of probe diffusion through fibrous networks to the case of a pr obe diffusing through a semi-dilute polymer inhomogeneous on the scale of t he polymer correlation length. The resulting exponential retardation agrees almost quantitatively with that found in recent electrophoresis experiment s of small proteins in polymer solutions that have been ascertained to be s emi-dilute (S. P. Radko and A. Chrambach, Electrophoresis, 17:1094-1102, 19 96; Biopolymers, 4:183-189, 1997).