Collective Membrane Dynamics under Osmotic Stress

Abstract

Phospholipid membranes are highly dynamic, ordered structures that involve molecular motions of phospholipids together with collective fluctuations of the bilayer [1]. Membrane structural dynamics on these length scales are sensitive to changes in properties such as temperature, pressure, and chemical potential. Structural deformation accompanying the removal of water from the membrane is well characterized, yet perturbation of membrane dynamics under osmotic stress conditions has not been studied. Here we show that membrane dynamics as revealed by 2H NMR relaxation measurements are sensitive to osmotic stress. Specifically, we measured the segmental order parameters (SCD) and 2H spin-lattice relaxation rates (R1Z) over a broad range of hydration levels. Empirical correlations of acyl chain SCD and R1Z profiles follow a theoretically predicted square-law functional dependence. However, for a given acyl position R1Z is essentially independent of SCD as the hydration water is varied. This is expected if the correlation length of the collective and segmental fluctuations remains unperturbed. The fast segmental fluctuations are decoupled from larger amplitude lipid motions within the osmotically stressed membrane. This result contrasts with studies involving cholesterol, where variations of SCD on the order of those observed in the osmotic stress experiment lead to significant reductions in R1Z rates [2]. In this case, interaction with cholesterol couples local segmental dynamics to collective viscoelastic modes. These results show that the relation of SCD to R1Z is a characteristic marker of lipid matrix composition and collective lipid interactions. Furthermore, our results highlight intrinsic differences in the sensitivity of membrane dynamics, as may be encountered for peripheral protein-membrane interactions and integral membrane-lipid interactions. [1] M.F. Brown and S.I. Chan, Encyclopedia of Nuclear Magnetic Resonance, Wiley, New York 1996, 871-885. [2] G.V. Martinez et al. (2004) Langmuir20,1043-1046.