Diffusion and partitioning of macromolecules in casein microgels: evidence for size-dependent attractive interactions in a dense protein system.

Understanding the mechanisms that determine the diffusion and interaction of macromolecules (such as proteins and polysaccharides) that disperse through dense media is an important fundamental issue in the development of innovative technological and medical applications. In the current work, the partitioning and diffusion of macromolecules of different sizes (from 4 to 10 nm in diameter) and shapes (linear or spherical) within dispersions of casein micelles (a protein microgel) is studied. The coefficients for diffusion and partition are measured using FRAP (fluorescence recovery after photobleaching) and analyzed with respect to the structural characteristics of the microgel determined by the use of TEM (transmission electron microscopy) tomography. The results show that the casein microgel displays a nonspecific attractive interaction for all macromolecules studied. When the macromolecular probes are spherical, this affinity is clearly size-dependent, with stronger attraction for the larger probes. The ...