Depletion interactions in the protein limit: Effects of polymer density fluctuations

We report the first observations of nonmonotonic changes in the second viri al coefficients of protein solutions, B-2, as the concentration of nonadsor bing polymer is increased. The observed minimum in B-2 cannot be predicted from standard depletion interaction energy models and is closely associated with proximity to the lower critical solution temperature of the polymer s olution. The location, depth, and molecular weight dependence of the minima are captured by the thermal polymer reference interaction sire model for d epletion interactions, where the polymer mesh size is a function of tempera ture.