Area Per Lipid and Elastic Deformation of Membrane Bilayers under Osmotic Stress

Abstract

The energetics of membrane proteins and cellular functions are modulated significantly by membrane properties [1]. Accurate knowledge of cross-sectional area/lipid of membrane systems is pertinent for molecular dynamics simulations [2,3]. Here we address the sensitivity of lipid bilayer structure and fluctuations to osmotic pressure and temperature using solid-state 2H NMR spectroscopy. Applied stress allows us to probe intermembrane interactions including collective membrane motions and lipid protrusions [1,3]. Through 2H NMR studies [3] we obtain striking evidence for bilayer deformation due to application of osmotic pressure to model membranes (DMPC-d54) using polyethylene glycol as osmolyte. The average structure of the membrane is manifested in the segmental order parameters (SCD) of the lipids that are measured with 2H NMR spectroscopy. Measurements of membrane structural parameters such as bilayer thickness and area per lipid employ a mean-torque analysis of 2H NMR order parameters (SCD) [4]. These NMR measurements allow us to interpret the free energy cost of bilayer deformation when combined with complementary X-ray observables. NMR results are used to calculate the osmotic coefficient to distinguish the different regimes of intermolecular forces, thereby yielding insights into bilayer separation forces [2]. We calculated the elastic area compressibility modulus that describes the membrane deformation with osmotic pressure. The present NMR study distinguishes between different intermembrane forces, and suggests that the undulations dominate at intermediate intermembrane distances whereas protrusions act at short distances. The thermodynamic descriptions of these experimental measurements shed light on the effect of osmotic pressure on membranes and their implications for protein functions. [1] M.F. Brown et al. (2002) JACS 124,8471-8484. [2] K.J. Mallikarjunaiah et al. (2011) BJ 100, 98-107. [3] H.I. Petrache et al. (2000) BJ79, 3172-3192. [4] K.J. Mallikarjunaiah et al. (2012) to be submitted to PCCP.