Urea-induced drying of hydrophobic nanotubes: comparison of different urea models.

In a previous study, we performed the molecular dynamics (MD) simulations of various carbon nanotubes solvated in 8 M urea and observed a striking phenomenon of urea-induced drying of hydrophobic nanotubes, which resulted from the stronger dispersion interaction of urea than water with nanotube (Das, P.; Zhou, R. H. J. Phys. Chem. B2010, 114, 5427-5430). In this paper, we have compared five different urea models to investigate if the above phenomenon is sensitive to the urea models used. We demonstrate through MD simulations that the drying phenomenon and its physical mechanism are qualitatively independent of the urea models. Consistent with our previous study, our current analyses with both interaction potential energy and association free energy indicate that there is a "dry state" inside the carbon nanotubes, which is caused by the urea's preferential binding to nanotubes through stronger dispersion interactions. These results also have implications for understanding the urea-induced protein denaturation by providing further evidence of the potential existence of a "dry globule"-like transient state during protein unfolding and the "direct interaction mechanism" (whereby urea attacks protein directly, rather than disrupts water structure as a "water breaker"). In addition, our study highlights the crucial role of dispersion interaction in the selective absorption of molecules in hydrophobic nanopores and may have significance for nanoscience and nanotechnology.