Dr Jackie Knobloch, Supervisor Dr Ingo Koeper
Neutron reflectometry is a technique used to study the structure of interfaces and thin films on a sub-molecular scale. It was initially employed to study thin inorganic films but is now used to study biological systems such as the interaction of proteins with cellular membranes, the adsorption and swelling of polymer films and the magnetic properties of multilayered devices. Reflectometry involves the specular reflection of a collimated beam of neutrons incident at grazing angles from a flat and smooth surface. The intensity of the reflected beam varies as a function of angle or wavelength depending on the structure, composition and roughness of the interface or thin film system.
Neutrons have many properties that are highly complimentary to those of X-rays. As they interact with the atomic nucleus, they are highly penetrating and can reach buried interfaces, for example, solid-liquid and liquid-liquid interfaces or samples that require bulky auxiliary environments such as a high pressure cell or magnet. This property also means neutrons are non-destructive to biological materials. One of the main benefits of using neutrons is their ability to detect low atomic weight elements and distinguish between elements close in atomic number. In particular, the difference in neutron scattering length between 1H (-3.74 fm) and 2H (6.67 fm) is taken advantage of in neutron scattering techniques. Two common examples of contrast variation are commonly seen. Firstly, isotopic substitution of H for D in molecules allows the location of molecules or particular chemical moieties in the system to be determined. The second use of contrast variation is the ability to hide or show different structures in a system by adjusting the ratio of deuterated solvent to hydrogenous solvent, commonly H2O to D2O, known as contrast matching. For example, the hydrocarbon chains of a phospholipid liposome can be hidden by matching the neutron scattering length density of the solvent so that only the head groups of the system are seen.
Figure 1. Left: Relative neutron and X-ray scattering cross sections. Right: Schematic diagram of specular reflection from a flat and smooth surface.
Neutron reflectometry to study the effect of anaesthetics on cellular membranes
Recently, we investigated the effect of ethanol, as a model anaesthetic, on model cellular membranes using the Platypus time of flight reflectometer at the Australian Nuclear Science and Technology Organisation (http://www.ansto.gov.au/ResearchHub/Bragg/). A tethered phospholipid bilayer was exposed to varying amounts of ethanol and the effect on the structure of the bilayer was measured. Deuterated ethanol was used to increase the contrast against the hydrogenous hydrocarbon chains of the phospholipids. A small change in the reflectivity was observed after exposing the bilayer to 10% v/v ethanol, see Figure 2, was but this effect could be reversed by rinsing the bilayer. At 50% v/v the effect was more apparent and the bilayer could not be recovered by rinsing.
Figure 2. Neutron reflectivity from a tethered phospholipid bilayer exposed to 10% v/v (left) and 50% v/v (right) deuterated ethanol.
- Schulz, J. C.; James, M.; Nelson, A.; Brule, A., Platypus: a time-of-flight neutron reflectometer at Australia’s new research reactor. Journal of Neutron Research 2006, 14 (2), 91-108.
- James, M.; Nelson, A.; Holt, S. A.; Saerbeck, T.; Hamilton, W. A.; Klose, F., The multipurpose time-of-flight neutron reflectometer “Platypus” at Australia’s OPAL reactor. Nucl. Instrum. Methods Phys. Res. Sect. A-Accel. Spectrom. Dect. Assoc. Equip. 2011, 632 (1), 112-123.