title: On the Role of Nonspherical Cavities in Short Length-Scale Density Fluctuations in Water.
abstract: Density fluctuations in liquid water are at the heart of numerous phenomena associated with hydrophobic effects such as protein folding and the interaction between biomolecules. One of the most fundamental processes in this regard is the solvation of hydrophobic solutes in water. The vast majority of theoretical and numerical studies examine density fluctuations at the short length scale focusing exclusively on spherical cavities. In this work, we use both first-principles and classical molecular dynamics simulations to demonstrate that density fluctuations in liquid water can deviate significantly from the canonical spherical shapes. We show that regions of empty space are frequently characterized by exotic, highly asymmetric shapes that can be quite delocalized over the hydrogen bond network. Interestingly, density fluctuations of these shapes are characterized by Gaussian statistics with larger fluctuations. An important consequence of this is that the work required to create non spherical cavities can be substantially smaller than that of spheres. This feature is also qualitatively captured by the Lum-Chandler-Weeks theory. The scaling behavior of the free energy as a function of the volume at short length scales is qualitatively different for the nonspherical entities. We also demonstrate that nonspherical density fluctuations are important for accommodating the hydrophobic amino acid alanine and are thus likely to have significant implications when it comes to solvating highly asymmetrical species such as alkanes, polymers, or biomolecules.
authors.affiliation: Thomas Young Centre, London Centre for Nanotechnology and Department of Physics and Astronomy, University College London , Gower Street, London WC1E 6BT, United Kingdom.
authors.forename: Gabriele Cesare
authors.identifier: http://orcid.org/0000-0002-6156-7399
authors.initials: GC
authors.lastname: Sosso
authors.affiliation: Department of Chemistry, University of Zurich , Winterhurerstrasse 190, Zurich CH-8057, Switzerland.
authors.forename: Sebastiano
authors.initials: S
authors.lastname: Caravati
authors.affiliation: Biophysics Graduate Group, University of California , Berkeley, California 94720, United States.
authors.forename: Grant
authors.initials: G
authors.lastname: Rotskoff
authors.affiliation: Department of Chemistry, The University of Chicago , Chicago, Illinois 60637, United States.
authors.forename: Suriyanarayan
authors.initials: S
authors.lastname: Vaikuntanathan
authors.affiliation: Condensed Matter and Statistical Physics Section, The Abdus Salam International Centre for Theoretical Physics , I-34151 Trieste, Italy.
authors.forename: Ali
authors.initials: A
authors.lastname: Hassanali