A11 Neutron Reflection

As many biological processes occur at interfaces (Fragneto-Cusani 2001), an understanding of the structure of proteins as they function at the solidliquid interface is very much sought after. One technique that has been used isneutronreflection. Thishas, overthelast25years, beenshowntobeavery valuable technique for investigating inhomogeneities across such interfaces

(Fragneto-Cusani 2001). Data at the interface is obtained by measuring the reflected intensity of a well-collimated neutron beam in comparison to the excitation intensity. By comparing these measurements at a range of incident angles and excitation wavelengths, the concentration profile at the interface can be calculated. Given the short wavelengths of neutron radiation, the dimensions of the layer perpendicular to the interface can be obtained at sub-nanometre (i.e. molecular) resolution. In addition, the method can provide exquisite detail where it is possible to deuterate materials at the interface, as contrast variation can then be used to probe the cross-section of layers formed at the interface. The technique has not, however, been widely adopted by the life science community. This is due to the fact that the neutron reflection facilities, whilst having increased significantly worldwide, are remote centralised facilities, and the cost of access remains high. In addition, the temporal resolution is currently of the order of a minute or so (Bucknall et al. 1999), which is not sufficient to monitor the early stages of structural changes that proteins typically undergo. This is, however, a rapidly developing area and, as such, this limitation may be addressed.

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