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Hydrophobicity operates over many scales, from the demixing of oil and water at the macroscopic scale to the folding of proteins in water at the molecular scale. The physics governing hydrophobicity at the two length scales are, however, fundamentally different (1). The hydration of large solutes is governed by surface tension, which favors lower surface area and causes oil drops to coalesce. The surface tension decreases monotonically with increasing temperature, and so does the driving force for coalescence. In contrast, at the microscopic scale, hydrophobic effects vary nonmonotonically, typically becoming stronger and reaching a maximum before decreasing with increasing temperature (e.g., folded proteins can be denatured both by heating and cooling, which implies a maximum in stability as a function of temperature). Theory and simulations predict that the crossover from the molecular to the macroscopic regime occurs at a length scale of the order of 1 nm (1). Because the relevant length scales in proteins range from subnanometer (for side chains exposed in their unfolded states) to several nanometers (in their folded states), understanding of hydration in the crossover region is important for estimating the hydrophobic driving forces in protein folding. Experimental measurements on hydrophobicity in this region have been elusive until now. In PNAS, Li and Walker (2) use single-molecule force spectroscopy of hydrophobic polymers to provide an experimental window into the crossover region. The length-scale dependence of hydrophobicity is, indeed, one of its most fascinating aspects. The difference in the physics of hydration of small and large solutes arises from the different manner in which they affect the structure of water (1, 3⇓⇓⇓⇓–8). Water molecules participate in a locally tetrahedral hydrogen bond network. Thermal fluctuations of that network can accommodate a small hydrophobic solute (9) without …

Unraveling the hydrophobic effect, one molecule at a time