Depletion Interactions in the Protein Limit: Effects of Polymer Density Fluctuations

We report the first observations of nonmonotonic changes in the second virial coefficients of protein solutions, $B_2$, as the concentration of nonadsorbing 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 solution. The location, depth, and molecular weight dependence of the minima are captured by the thermal polymer reference interaction site model for depletion interactions, where the polymer mesh size is a function of temperature.