Computational probe of cavitation events in protein systems.

Previous all-atom simulations have identified several classes of proteins where hydrophobic de-wetting (cavitation) is at play. Here we develop and validate a computationally fast method that predicts in which protein systems water spontaneously cavitates. We implement a cubic lattice model, which incorporates the protein shape from crystallographic data and the protein-water interactions from thermodynamic data. Combining it with the previously developed coarse-grained model for water, we determine the extent of occupancy of water at protein-protein interfaces and in protein-ligand cavities. The model captures essential findings from all-atom molecular dynamics studies on the same systems by distinguishing protein cavities that dry from those that remain wet. We also interpret the origin of the cavitation inside the melittin tetramer on simple thermodynamic grounds, and show that part of the mellitin surface is sufficiently hydrophobic to trigger cavitation. Using Glauber/Kawasaki dynamics we obtain the time-scales for de-wetting events that are in agreement with those from all-atom simulations. The method can serve as an intermediate step between the necessary initial screening that identifies proteins with abundance of hydrophobic patches using bioinformatics tools (L. Hua, X. H. Huang, P. Liu, R. H. Zhou and B. J. Berne, J. Phys. Chem. B, 2007, 111, 9069), and computationally extensive studies that need to incorporate molecular details (e.g. single mutation studies of amino acid residues).

The influence of molecular-scale roughness on the surface spreading of an aqueous nanodrop.

We examine the effect of nanoscale roughness on spreading and surface mobility of water nanodroplets. Using molecular dynamics, we consider model surfaces with sub-nanoscale asperities at varied surface coverage and with different distribution patterns. We test materials that are hydrophobic, and those that are hydrophilic in the absence of surface corrugations. Interestingly, on both types of surfaces, the introduction of surface asperities gives rise to a sharp increase in the apparent contact angle. The Cassie-Baxter equation is obeyed approximately on hydrophobic substrates, however, the increase in the contact angle on a hydrophilic surface differs qualitatively from the behavior on macroscopically rough surfaces described by the Wenzel equation. On the hydrophobic substrate, the superhydrophobic state with the maximal contact angle of 180 degrees is reached when the asperity coverage falls below 25%, suggesting that superhydrophobicity can also be achieved by the nanoscale roughness of a macroscopically smooth material. We further examine the effect of surface roughness on droplet mobility on the substrate. The apparent diffusion constant shows a dramatic slow down of the nanodroplet translation even for asperity coverage in the range of 1% for a hydrophilic surface, while droplets on corrugated hydrophobic surfaces retain the ability to flow around the asperities. In contrast, for smooth surfaces we find that the drop mobility on the hydrophilic surface exceeds that on the hydrophobic one.