Is the depletion force entropic? Molecular crowding beyond steric interactions

Abstract

Cosolutes excluded from macromolecules create effective attractions between the excluding macromolecules, and promote their self-association. This “depletion force” serves an important stabilizing role in many biological and technological processes. Specifically, many osmolytes and polymeric crowders that are excluded from protein surfaces stabilize the more compact folded state. Yet, these excluded cosolutes operate through several, thermodynamically distinct, mechanisms. Here we review the experimentally found mechanisms and link these to possible underlying molecular interactions. One class of excluded cosolutes comprises protective osmolytes. These tend to induce attractive depletion forces that are enthalpically dominated but entropically destabilizing. Whereas polymeric crowders may act by similar mechanisms to osmolytes, in many other cases they induce depletion forces dominated by favorable entropy. These different thermodynamic mechanisms are intimately related to the effective interaction of cosolute with each macromolecule. The venerable Asakura-Oosawa model for depletion forces assumes that the cosolute-macromolecule interaction is entirely steric, thereby predicting fully entropic depletion forces. Augmenting this steric repulsion core with a “soft” repulsion shell adds an enthalpic contribution to the depletion force, which may even dominate all other contributions. Further, considering that cosolute-macromolecule effective interactions are temperature-dependent results in a depletion force that can be concertedly dominated by enthalpy and entropically disfavored, as observed experimentally for protective osmolytes. In this core-shell description, to account for depletion forces that are enthalpically dominated and entropically disfavored, it is sufficient to consider an effective microscopic cosolute-macromolecule soft shell that comprises an entropic attraction and an enthalpic repulsion. We show how the full gamut of cosolute effects can be rationalized using these simple considerations regarding the nature of the cosolute-macromolecule effective interaction.