Restructuring of Membrane Bilayers due to Osmotic Pressure: Deuterium Solid-State NMR Study

Abstract

Lipid membrane composition and biophysical properties have substantial influences on cellular functions. Studies of environmental effects on membrane bilayers are a prerequisite for understanding membrane protein functions. Experimental measures of structural parameters like cross-sectional area/lipid of membrane bilayers are vital for molecular dynamics simulations [1,2]. We used solid-state 2H NMR spectroscopy which gives site-specific orientational order parameters of the lipid segments to study structural fluctuations and deformations of phospholipid bilayers due to osmotic stress [1]. Model lipid membrane systems (DMPC and POPC) were subjected to osmotic pressure and temperature to establish their sensitivity to environmental changes in the liquid-crystalline state. Osmotic stress was applied by addition of osmolytes (polyethylene glycol) as well as by gravimetric dehydration. We observed very large changes in segmental order parameters with the application of osmotic pressures in the biological range. The NMR order parameters represent the area/lipid and show large changes in mean-square fluctuations of the lipid structure [3,4]. Stresses from these pressures are thermodynamically equivalent because changing chemical potential when transferring water from the interlamellar space to the bulk water phase corresponds to an induced pressure, as verified experimentally [1]. By employing mean-torque analysis of the NMR observables [3] we calculated the mean area per lipid and the volumetric bilayer thickness, which change up to 20% upon introduction of osmotic stress. These 2H NMR studies [4] show striking bilayer deformation due to the application of osmotic pressure. They distinguish molecular-level force regimes associated with lipids that can play a significant role in biological processes.[1] K.J. Mallikarjunaiah et al. (2011) BJ 100, 98-107. [2] A. Leftin et al. (2011) BBA1808, 818-839. [3] H.I. Petrache et al. (2000) BJ79, 3172-3192. [4] K.J. Mallikarjunaiah et al. (2012) to be submitted to PCCP.