Remarkable solvent effects on depletion interaction in crowding media: Analyses using the integral equation theories

In theoretical studies on the depletion interaction between macromolecules in crowding media, usually only the macromolecules and large cosolutes are considered by assuming that the solvent can be treated as an inert background. Here, the interaction between macromolecules immersed in a solvent-cosolute mixture is calculated using the integral equation theories combined with a hard-sphere model. On the basis of the results, we point out the crucial importance of explicit incorporation of solvent molecules in the theoretical treatment. The behavior observed is highly nonlinear due to the complicated intermolecular correlations and remarkably dependent on the total packing fraction and the cosolute concentration. An example result is that the cosolute can act both as a stabilizer and as a destabilizer for a dimer of macromolecules. We present new physical pictures elucidating the behavior.