Neutron and X-ray scattering for biophysics and biotechnology: examples of self-assembled lipid systems

Abstract

Membranes that surround cells and separate their contents from the external environment are ubiquitous in biological systems. These membranes are organized assemblies consisting mainly of lipids and proteins, and are highly selective permeability barriers which control the flow of information between cells and their environment. It is accepted that the lipid bilayer is the underlying structure of most, if not all, biomembranes. As such, over the years scientists have exerted much effort in studying lipid bilayers and their biological relevance in hopes of understanding the functional mechanisms taking place at membrane interfaces. Neutron and X-ray scattering techniques are powerful tools for the characterization of the structure and dynamics of biomimetic systems as they provide unique access to microscopic structure and dynamics at length scales ranging from microns to intermolecular and/or atomic distances. The optimization of instruments and preparation techniques, as well as the new possibilities offered by protein deuteration, have opened up new avenues for the study of lipid/protein interactions that were not previously possible. One can now look at the insertion of biomolecules into membranes and accurately determine the structure as well as the dynamics of the interaction. To illustrate the usefulness of diffraction and scattering techniques with regard to biologically relevant systems, we review some of the leading edge studies that have taken place over the last couple of years in which these scattering techniques have played a central role.

Cation-containing lipid membranes – experiment and md simulations

“…We used a different approach to analyse the experimental data, producing more quantitative results. This approach evaluates all the different contrast diffraction data simultaneously, while assuming that water probability follows the error function (Kučerka et al, 2009). Results in Fig.…”

“…1). In addition, the contrast varied diffraction data can be subtracted from each other, providing water probability distributions similar to those obtained from MD simulations (Kučerka et al, 2009). Fig.…”

“…The rule arises from the fact that for centro-symmetric systems (as those studied here) scattering intensity is proportional to the square of the contrast between the measured object and the solvent, (Worcester et al, 1976). Once the form factors and their signs are determined, the neutron scattering length density (NSLD) profile is calculated by Fourier transformation (Kučerka et al, 2009). Except for the assumption that the bilayers are centrosymmetric, this procedure inverts the data from reciprocal space into real space without any further assumptions.…”

“…The diffraction curves were taken using standard symmetric diffraction scans, where scattering space was probed along the z direction of the scattering vector q. The diffraction curves allow for the reconstruction of one-dimensional neutron scattering length density (NSLD) profiles, as outlined in an earlier study (Kučerka et al, 2009). In addition, rocking curves were collected to evaluate the quality of sample orientation (Nagle et al, 2016 …”

Biomembranes research using thermal and cold neutrons

“…These developments inspired the construction of facilities for the large-scale production of neutrons, subsequently creating new areas of research. Large quantities of neutrons suitable for scattering experiments are produced either by the fission of uranium-235 (e.g., ORNL's 85 MW High Flux Isotope Reactor HFIR) or by spallation, where high velocity particles (e.g., protons) produced by an accelerator impinge on a heavy metal target (liquid mercury in the case of ORNL's Spallation Neutron Source, SNS) (Harroun, Kucerka et al 2009). The resulting fast neutrons are slowed ('thermalized') by passing through and interacting with a moderator (e.g., H 2 O, D 2 O, graphite, Be).…”