Exploring the Local Elastic Properties of Bilayer Membranes Using Molecular Dynamics Simulations Gilles Pieffet, †,§ Alonso Botero, † Gü nther H. Peters, ‡ Manu Forero-Shelton, † and Chad Leidy* ,† † Department of Physics, Universidad de los Andes, Carrera 1 No 18A 10, Bogota ́ , Colombia ‡ Department of Chemistry, Technical University of Denmark, 2800 Kongens Lyngby, Denmark * S Supporting Information ABSTRACT: Membrane mechanical elastic properties regulate a variety of cellular processes involving local membrane deformation, such as ion channel function and vesicle fusion. In this work, we used molecular dynamics simulations to estimate the local elastic properties of a membrane. For this, we calculated the energy needed to extract a DOPE lipid molecule, modified with a linker chain, from a POPC bilayer membrane using the umbrella sampling technique. Although the extraction energy entails several contributions related not only to elastic deformation but also to solvation, careful analysis of the potential of mean force (PMF) allowed us to dissect the elastic contribution. With this information, we calculated an effective linear spring constant of 44 ± 4 kJ·nm -2 ·mol -1 for the DOPC membrane, in agreement with experimental estimates. The membrane deformation profile was determined independently during the stretching process in molecular detail, allowing us to fit this profile to a previously proposed continuum elastic model. Through this approach, we calculated an effective membrane spring constant of 42 kJ·nm -2 · mol -1 , which is in good agreement with the PMF calculation. Furthermore, the solvation energy we derived from the data is shown to match the solvation energy estimated from critical micelle formation constants. This methodology can be used to determine how changes in lipid composition or the presence of membrane modifiers can affect the elastic properties of a membrane at a local level. ■ INTRODUCTION Membrane elastic properties are known to regulate the activity of integral membrane proteins by altering the energy cost of membrane deformation associated with protein conformational changes. 1-5 Ion channel dimerization and vesicle fusion depend strongly on these elastic properties, 1-3 which are influenced by changes in membrane composition through the introduction of lipid metabolites such as lysophospholipids, 6 cholesterol, 7 and polyunsaturated fatty acids. 8 For example, the presence of cholesterol has been shown to significantly increase the bending rigidity of the membrane for particular lipid species. 7 A key parameter is the effective spring constant, which is determined by the energy required to stretch the membrane in a direction normal to the surface. The effective spring constant has been used to characterize the energy contribution due to the insertion of a protein inclusion presenting a hydrophobic mismatch with the membrane, 9,10 for example, in studies of gramicidin dimerization 3 and its modulation. 11 These elastic properties are normally measured experimen- tally using micropipette aspiration 12-17 or fluctuation anal- ysis 18,19 or can be estimated through X-ray diffraction. 20-22 However, these experimental approaches provide only average global elastic behavior and cannot be used to investigate elastic behavior at a local level. In this work, we estimated the effective spring constant at a local level for a membrane using molecular dynamics (MD) simulations and umbrella sampling (US) simulations. 23 We chose a 1-palmitoyl-2-oleoylphosphatidylcholine (POPC) membrane in which we included a 1,2-dioleoylphosphatidyle- thanolamine lipid molecule modified with a linker chain (mDOPE) as the inclusion being pulled. This was meant to ensure consistency with future force spectroscopy pulling experiments using an avidin-biotin-coupled system. A potential of mean force (PMF) calculation of a lipid extraction process on a different membrane system was previously reported. 24 However, the authors focused on determining the energetics of desorption, and the elastic properties of the membrane were not considered. From our simulations, we were also able to estimate the energy cost of pulling the mDOPE lipid molecule from a POPC bilayer membrane and separate the energy contributions from membrane deformation and solvation. Alternatively, we calculated the deformation energy by means of the continuum elastic model 2,4 using the average deformation of the membrane at different distances of pulled mDOPE, and we compare the resulting effective membrane spring constant with the one calculated from the PMF profile. We expect that our methodology will provide a useful tool for estimating the energy contribution to the mechanical properties of integral membrane proteins resulting from local Received: May 5, 2014 Revised: October 17, 2014 Published: October 17, 2014 Article pubs.acs.org/JPCB © 2014 American Chemical Society 12883 dx.doi.org/10.1021/jp504427a | J. Phys. Chem. B 2014, 118, 12883-12891