Membrane Area Deformation under Osmotic Stress: Deuterium NMR Approach

Abstract

We address the hypothesis that the sensitivity of lipid bilayers to pressure, temperature, and osmotic stress represents influences of non-specific lipid-protein interactions on functions of cellular membranes [1,2]. Measurements of membrane structural parameters such as bilayer thickness and area per lipid employ a mean-torque analysis [3] of 2H solid-state NMR order parameters (SCD). NMR lipid order parameters are very sensitive to changes in cross-sectional area per molecule. We observed striking (≈20%) changes in structural properties (decrease in area per lipid and increase in bilayer thickness) when ≈200 atmospheres of pressure (dehydration pressure or osmotic pressure) are applied to lipid (DMPC) bilayers in the liquid-crystalline state. We show the equivalence of osmotic and dehydration pressures by NMR experimental measurements as well as thermodynamically [2]. The elastic area compressibility modulus (Ka) of bilayers is determined by employing different pressure techniques in combination with NMR and vapor pressure osmometry methods. Our findings agree well with the reported Ka value for DMPC [4] obtained with a much smaller range of osmotic pressures. However, we observe an additional variation in Ka determined at higher osmotic pressures, where the role of complex dynamics in the bilayer structural changes becomes more evident. We propose that the hierarchy of forces and motions is perturbed by membrane dehydration (osmotic pressure) due to the alteration of interlamellar spacings, with corresponding changes in elastic area compressibility moduli. Our findings have significant implications for the applicability of solid-state 2H NMR spectroscopy together with membrane stress techniques for understanding the mechanisms of action of pressure-sensitive proteins.[1] A.V. Botelho et al. (2006) BJ 91, 4464-4477.[2] K.J. Mallikarjunaiah et al. (2011) BJ 100, 98-107.[3] H.I. Petrache et al. (2000) BJ 79, 3172-3192.[4] H.I. Petrache et al. (1998) CPL 95, 83-94.