Protein-induced collapse of polymer chains

The collapse of polymer chains in a good solvent due to the presence of small colloidal spheres, taken to model inert globular proteins, is discussed. It is argued that the large-scale properties of the chains in the mixture are adequately described using Edwards' type (delta function) interactions between all constituents, provided that the colloidal particles are very much smaller than the polymer coils and that their concentrations are not too high. Chain collapse occurs when the mutually excluded volumes of the segments, renormalized by the concentration fluctuations of the colloids, become negative. A closed expression for the colloid density at which the collapse transition should be expected is given and is compared with recently published computer simulation results. The agreement is reasonably good, even at densities which are outside of the range of validity of the theory.