Mastering contrast in structural studies usingNetron scattering from neutron scattering

What is it about?

Hydrogen is most abundant in living systems and in large number of materials, like polymers. There is no method as versatile for the study of hydrogenous substances like neutron scattering. This is so for two reasons. First, the interaction of neutrons with nuclei changes significantly with the isotope. The replacement of 1H by 2H is a classical example. An even more important change of the change of the scattering length is observed with polarised neutron scattering from polarised nuclei. Again, this property is dependent on the isotope as well. It is most spectacular with the nuclei of 1H (protons). What we need is to get the interacting particles polarised. While polarised protons are available at almost any powerful neutron source, the polarisation of nuclear spins requires Dynamic Nuclear Polarisation (DNP) a technique typically used in experiments of High Energy Physics.

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Photo by Martin Vysoudil on Unsplash

Photo by Martin Vysoudil on Unsplash

Why is it important?

The isotopic substitution of 1H by 2H adds a new dimension to structural studies. The gradual change of the content of heavy water in an aqueous medium is a relatively cheap and most efficient way to elucidate the internal structure of macromolecules. The main constituents of biomolecules, like lipids, RNA/DNA and proteins are easily identified and localized. Similar aspects apply to studies in materials science. They hold for both crystallography ans small-angle scattering from solutions. There is a clear trend towards selective deuteration of parts of a maromolecule. The structural studies of the ribosome using the triangulation method are a well-known example. The results were superseded by crystallographic studies using synchrotron radiation. But, note that there is an undeniable progress in using neutron crystallography, as well. The use of polarised nuclei in crystallography is promising but so far rare. Most of the studies rely on small-angle scattering from hydrogenous materials, particularly in Japan. A special type concerns time-resolved neutron scattering aiming at large scale movements of the proton polarisation. The elucidation of sources and sinks of the proton polarisation in radical proteins is a recent example.

Perspectives