Single Polymer Studies of Hydrophobic Hydration

Hydrophobic interactions guide protein folding, multidomain protein assembly, receptor-ligand binding, membrane formation, and cellular transportation. On the macroscale, hydrophobic interactions consist of the aggregation of "oil-like" objects in water by minimizing the interfacial energy. However, studies of the hydration behavior of small hydrophobic molecules have shown that the microscopic (~1 nm) hydration mechanism differs fundamentally from its macroscopic counterpart. Theoretical studies over the last two decades have pointed to an intricate dependence of molecular hydration mechanisms on the length scale. The microscopic-to-macroscopic crossover length scale is critically important to hydrophobic interactions in polymers, proteins, and other macromolecules. Accurate experimental determination of hydration mechanisms and interaction strengths directly influence our understanding of protein folding. In this Account, we discuss our recent measurements of the hydration energies of single hydrophobic homopolymers as they unfold. We describe in detail our single molecule force spectroscopy technique, the interpretation of the single polymer force curve, and how it relates to the hydration free energy of a hydrophobic polymer. Specifically, we show how temperature, side-chain sizes and solvent conditions, affect the driving force of hydrophobic collapse. The experiments reveal that the size of the nonpolar polymer side-chains changes the thermal signatures of hydration. The sizes of the polymer side-chains bridge the length scale where theories had predicted a transition between entropically driven microscopic hydration and enthalpically driven macroscopic hydrophobic hydration. Our experimental results revealed a crossover length scale of approximately 1 nm, similar to the results from recent theoretical studies. Experiments that probe solvent dependency show that the microscopic polymer hydration is correlated with macroscopic interfacial tension. Consistent with theoretical predictions, the solvent conditions affect the microscopic and macroscopic hydrophobic strengths in similar ways. Although the extended polymers and proteins span hundreds of nanometers, the experiments show that their hydration behavior is determined by the size of a single hydrophobic monomer. As the hydrophobic particle size decreases from the macroscopic to the microscopic regime, the scaling relationship changes from a dependence on interfacial area to a dependence on volume. Therefore, under these conditions, the driving force for the aggregation of hydrophobic molecules is reduced, which has significant implications for the strength of hydrophobic interactions in molecular systems, particularly in protein folding.