Unravelling the structural complexity of protein–lipid interactions with neutron reflectometry

Abstract

Neutron reflectometry (NR) is a large-facility technique used to examine structure at interfaces. In this brief review an introduction to the utilisation of NR in the study of protein–lipid interactions is given. Cold neutron beams penetrate matter deeply, have low energies, wavelengths in the Ångstrom regime and are sensitive to light elements. High differential hydrogen sensitivity (between protium and deuterium) enables solution and sample isotopic labelling to be utilised to enhance or diminish the scattering signal of individual components within complex biological structures. The combination of these effects means NR can probe buried structures such as those at the solid–liquid interface and encode molecular level structural information on interfacial protein–lipid complexes revealing the relative distribution of components as well as the overall structure. Model biological membrane sample systems can be structurally probed to examine phenomena such as antimicrobial mode of activity, as well as structural and mechanistic properties peripheral/integral proteins within membrane complexes. Here, the example of the antimicrobial protein α1-purothionin binding to a model Gram negative bacterial outer membrane is used to highlight the utilisation of this technique, detailing how changes in the protein/lipid distributions across the membrane before and after the protein interaction can be easily encoded using hydrogen isotope labelling.

Probing the Separation Distance between Biological Nanoparticles and Cell Membrane Mimics Using Neutron Reflectometry with Sub-Nanometer Accuracy

“…65 In the case of SLBs, NR was not only successfully used to quantify minute differences in the thickness of hydrophobic and hydrophilic portions of SLBs made from different lipid compositions 66 but also structural alterations induced by changes in environmental conditions 67,68 and biomolecular interactions. 69 However, while thickness determination is relatively straightforward, separation distance determination is particularly challenging using NR, as NR requires strong SLD contrasts in the direction orthogonal to the interface and a well-defined layered structure, so far only realized for homogeneous flat layers. 12,70,71 Thus, due to the geometry of spherically shaped nanoparticles, NR has been rarely used for in-depth characterization of adsorbed nanoparticles.…”

“…In particular, aided by the possibility to vary the scattering length density (SLD) contrast , between the substrate, the adsorbed film, and the background buffer by exchanging protium ( 1 H) with deuterium ( 2 H), NR has been shown to offer absolute thickness determinations with sub-nanometer precision of biomolecular films and their internal structure, even for multilayered systems . In the case of SLBs, NR was not only successfully used to quantify minute differences in the thickness of hydrophobic and hydrophilic portions of SLBs made from different lipid compositions but also structural alterations induced by changes in environmental conditions , and biomolecular interactions . However, while thickness determination is relatively straightforward, separation distance determination is particularly challenging using NR, as NR requires strong SLD contrasts in the direction orthogonal to the interface and a well-defined layered structure, so far only realized for homogeneous flat layers. ,, Thus, due to the geometry of spherically shaped nanoparticles, NR has been rarely used for in-depth characterization of adsorbed nanoparticles.…”

Bax Forms a Membrane Surface Protein-Lipid Complex as it Initiates Apoptosis

“…4 A and 4 B show time-resolved NR data obtained during the interaction of (h-)Bax with the 15 mol% TOCL containing mitochondrial outer membrane model obtained in a D 2 O buffer solution. NR data sets obtained in D 2 O are sensitive to both the lipid and h-protein distributions across the surface (37). This data was optimally fitted to models which were a scale ratio between the surface structure before the interaction of Bax and the structure at equilibrium Bax binding.…”

Probing the Separation Distance between Biological Nanoparticles and Cell Membranes Mimics Using Neutron Reflectometry with Sub-Nanometer Accuracy

“…In particular, aided by the possibility to vary the contrast 61,62 between the substrate, the adsorbed film, and the medium by exchanging protium ( 1 H) with deuterium ( 2 H) atoms, NR has been shown to offer absolute thickness determinations with sub-nanometer precision of biomolecular films 63 and their internal structure, 64 as well as of multilayered systems. 65 In the case of SLBs, NR was not only successfully used to quantify minute differences in the thickness of hydrophobic and hydrophilic portions of SLBs made from different lipid compositions 66 but also the structural modifications induced by changes in environmental conditions 67,68 and biomolecular interactions 69 . However, while thickness determination is relatively simple to obtain, separation distance determination is particularly challenging using NR, as it requires strong contrasts in the orthogonal direction to the interface and a well-defined layered structure, so far only possible for homogeneous flat layers.…”