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Changes in Phosphatidylcholine Headgroup Tilt and Water Order Induced by Monovalent Salts: Molecular Dynamics Simulations

Jonathan N. Sachs ^*, Hirsh Nanda , Horia I. Petrache  and Thomas B. Woolf 

^* Department of Biomedical Engineering, Program in Molecular Biophysics, and Department of Physiology, The Johns Hopkins University School of Medicine, Baltimore, Maryland 21205; and Laboratory of Physical and Structural Biology, The National Institute of Child Health and Human Development, National Institutes of Health, Bethesda, Maryland 20892

Correspondence: Address reprint requests to Dr. Thomas B. Woolf, The Johns Hopkins University, Dept. of Physiology, 206 Biophysics Bldg., Baltimore, MD 21205.

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.

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