Interfacial water structure, peptide adsorption and ion-specificity

i) Hydrophobic surfaces in contact with water show a pronounced depletion layer with a thickness of a few Angstroms within which the water density is highly reduced[1]. This layer leads to unusual static and kinetic properties including a finite slip length[2]. ii) Many polypeptides readily adsorb on both hydrophobic and hydrophilic surfaces. Single molecule AFM studies yield adsorption energies and point to an extremely high mobility on hydrophobic surfaces. The dominant hydrophobic attraction can be quantitatively explained with classical MD simulations including explicit water. Both water structural effects and dispersion interactions contribute to this solvation attraction[3]. iii) The friction coefficient of bound polymers is very low on hydrophobic substrates, which is traced back to the presence of a vacuum layer between substrate and water, which forms a lubricating cushion on which a polymer can glide. Conversely, friction forces on hydrophilic substrates are large and make determining the equilibrium binding constant in computer simulations impossible.[4] iv) Nominally uncharged, hydrophobic surfaces appear negatively charged and give rise to sizable electrostatic adsorption energies. This finding is related to the water structure at hydrophobic substrates which involves an oriented first water layer. Large anions (e.g. Iodide and Bromide) adsorb more strongly than small halides (Fluoride and Chloride) on hydrophobic surfaces, explaining the so-called Hofmeister series that is found for interfacial energies, surface potentials and interactions between solutes in general[5]. v) Modeling ion specificity involves reproducing ion solvation in water in the first place. Some recent progress and remaining obstacles with current Molecular Dynamics force fields are discussed.

[1] Interfacial Water at Hydrophobic and Hydrophilic Surfaces: Depletion versus Adsorp- tion, J. Janecek and R.R. Netz, Langmuir 23, 8417 (2007)
[2] Water slippage versus contact angle: a quasi-universal relationship, D.M. Huang, C. Sendner, D. Horinek, R.R. Netz, and L. Bocquet, Physical Review Letters (2008)
[3] Peptide adsorption on a hydrophobic surface results from an interplay of solvation, surface and intrapeptide forces, D. Horinek, A. Serr, M. Geisler, T. Pirzer, U. Slotta, S. Q. Lud, J. A. Garrido, T. Scheibel, T. Hugel, R. R. Netz, PNAS 105, 2842 (2008)
[4] Polypeptide friction and adhesion on hydrophobic and hydrophilic surfaces: A molecular dynamics case study, A. Serr, D. Horinek and R.R. Netz, Journal of the American Chemical Society 130, 12408 (2008)
[5] Specific Ion Adsorption at Hydrophobic Solid Surfaces, D. Horinek and R.R. Netz Physical Review Letters 99, 226104 (2007)