Hydrophobicity plays an important role in driving numerous biomolecular and other aqueous self-assemblies.However, characterizing the hydrophobicity of nanostructured solutes has remained a challenge because the chemical and topographical context presented by the solute to its hydration waters influences its hydrophobicity in a non-trivial manner. Using a combination of statistical mechanics and novel simulation techniques, we show that such context-dependent hydrophobicity of nanostructured solutes can be characterized by the free energetic cost of displacing water molecules from the vicinity of the solute; the easier it is to displace the hydration waters, the more hydrophobic the underlying surface.By quantifying such a the collective response of water near surfaces with systematic variations in curvature and chemical patterns, we first unocver how these surface characteristics influence hydrophobicity. In particular, we find that water density fluctuations are enhanced significantly near concave non-polar surfaces compared to those near flat or convex ones, suggesting that concave surfaces are more hydrophobic. We also find that patterned SAM surfaces, which have the same number of non-polar and polar sites, but differ only in their geometrical arrangements, can display dramatically different hydrophobicities. Specifically, hydropholic sites reduce surface hydrophobicity most effectively, not when they are clustered together, but when they are separated by at least one hydrophobic site. Similarly, studies on protein mutants show that a charged amino acid can dramatically reduce the hydrophobicity of a large non-polar patch when placed at the center of the patch, rather than at its edge. Our results highlight that popular simplifications, such as surface area models or hydropathy scales, which do not account for the precise chemical and topographical context presented by the solute, can not accurately predict solute hydrophobicity or its role in driving solute interactions. In contrast, water density fluctuations can serve as context- dependent measures for accurately characterizing the hydrophobicity of chemically and topographically complex solutes.

Understanding and characterizing the context-depending hydrophobicity of nanostructured solutes

254th National Meeting (2017)

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