The association between monovalent salts and neutral lipid bilayers is known to influence global bilayer structural properties such as headgroup conformational fluctuations and the dipole potential. The local influence of the ions, however, has been unknown due to limited structural resolution of experimental methods. Molecular dynamics simulations are used here to elucidate local structural rearrangements upon association of a series of monovalent Na+ salts to a palmitoyl-oleoyl-phosphatidylcholine bilayer. We observe association of all ion types in the interfacial region. Larger anions, which are meant to rationalize data regarding a Hofmeister series of anions, bind more deeply within the bilayer than either Cl- or Na +. Although the simulations are able to reproduce experimentally measured quantities, the analysis is focused on local properties currently invisible to experiments, which may be critical to biological systems. As such, for all ion types, including Cl-, we show local ion-induced perturbations to headgroup tilt, the extent and direction of which is sensitive to ion charge and size. Additionally, we report salt-induced ordering of the water well beyond the interfacial region, which may be significant in terms of hydration repulsion between stacked bilayers.
Bibliographical noteFunding Information:
Partial support of the research was from the American Cancer Society under grant RSG-01-048-01-GM.
The authors thank Dr. Alan Grossfield, Dr. Hyunbum Jang, and Naveen Michaud-Agrawal for helpful discussions, and Dr. Scott E. Feller for providing the POPC starting structure. J.N.S. thanks the Whitaker Foundation for Biomedical Engineering for graduate fellowship support, and the National Computational Science Alliance and Pittsburgh Supercomputer Center for a generous computational grant and supercomputer resources.