Neutron scattering studies on dynamics of lipid membranes

Abstract

Neutron scattering methods are powerful tools for the study of the structure and dynamics of lipid bilayers in length scales from sub Å to tens to hundreds nm and the time scales from sub ps to μs. These techniques also are nondestructive and, perhaps most importantly, require no additives to label samples. Because the neutron scattering intensities are very different for hydrogen- and deuterium-containing molecules, one can replace the hydrogen atoms in a molecule with deuterium to prepare on demand neutron scattering contrast without significantly altering the physical properties of the samples. Moreover, recent advances in neutron scattering techniques, membrane dynamics theories, analysis tools, and sample preparation technologies allow researchers to study various aspects of lipid bilayer dynamics. In this review, we focus on the dynamics of individual lipids and collective membrane dynamics as well as the dynamics of hydration water.

FIG. 1.

The length (d) and timescales (t), and the corresponding momentum (Q) and energy (E) transfers, covering the hierarchy of membrane dynamics. The range of interested length scales in lipid structure study is covered by a variety of scattering and microscopic techniques as shown in the bottom of the figure. The dynamic ranges of the spectroscopic techniques available to measure the different dynamics are also shown, with neutron techniques as light blue, x-ray in magenta, light in green, and fluorescence and optical imaging techniques in orange. Spectroscopic techniques, such as dielectric spectroscopy, muon spin resonance, nuclear magnetic resonance (NMR), and fluorescence correlation spectroscopy, access a broad range of timescales without any specific associated length scales. The lipid images in the figure is adapted with permission from Kelley et al., in Characterization of Biological Membranes: Structure and Dynamics, edited by M.-P. Nieh, F. A. Heberle, and J. Katsaras (De Gruyter, 2019), pp. 131–176.

FIG. 2.

Schematic drawing of lipid and water molecules and neutron scattering lengths. Different atoms have different neutron scattering lengths, which are represented by different colors in the figure. (a) Lipid molecules are protiated while water is deuterated; a suitable deuteration scheme to measure lipid dynamics by QENS. (b) Lipid molecules are deuterated while water is protiated; a preferable deuteration scheme to measure water dynamics by QENS. (c) The same deuteration scheme as in A, while the membrane is seen in larger length scales for NSE; a deuteration scheme to see the membrane dynamics as a whole. (d) Lipid acyl tails are deuterated and dispersed in deuterium oxide solvent and observation length scales are relatively long; sensitive to internal membrane dynamics such as thickness fluctuations.