Neutron Diffraction

Abstract

Neutron diffraction, a technique analogous to X‐ray diffraction, is well‐suited to the study of biological materials. By differentiating between hydrogen and its heavier isotope, deuterium, neutron diffraction is able to provide detailed information about the hydrogen atoms within biological macromolecules. It allows biologists to study solvation effects, protonation–deprotonation equilibria, hydrogen bonding and other types of chemistry, all of which are invisible to X‐ray techniques. Recent years have seen the construction of new neutron research facilities, notably the Spallation Neutron Source in the USA, the World's brightest neutron source. Even with such high‐flux sources, the intensity of neutrons hitting the sample is several orders of magnitude lower than a typical X‐ray experiment. However, the nonionizing nature of neutrons means that beam damage to the specimen is low, even over extended exposure times.

Key Concepts:

  • Neutron diffraction is closely related to the complementary technique of X‐ray crystallography, a method that led to a revolution in biological sciences.

  • Hydrogen plays a central role in many biological processes, but is invisible to X‐rays.

  • Neutrons are able to distinguish between hydrogen and deuterium, so selective deuteration of a single molecule or group can be used to locate it within a complex macromolecular assembly.

  • The neutron scattering contrast of samples can be controlled by changing the deuterium content of the solvent (water–heavy water exchange).

  • Neutrons have the ability to discriminate between nitrogen, carbon and oxygen, which can be difficult with X‐rays.

  • Neutron experiments are carried out at large scale, national or international neutron research centres, because production of the neutrons requires a nuclear reactor or a spallation source.

  • Animal behaviourists must participate in conservation planning to protect the future of biodiversity.

  • Lipid bilayers provide the fundamental architecture of biological membranes.

Keywords: neutron; hydrogen; deuterium; crystal; protein

Figure 1.

Neutron and X‐ray scattering lengths (10−12 cm) for the principal atoms of biomaterials. For X‐rays, the scattering length is proportional to the number of electrons. Neutrons are scattered by the nucleus of an atom, so the neutron scattering length varies with the different isotopes of each element. The neutron scattering length of deuterium (2H) is positive, whereas that of hydrogen (1H) is negative.

Figure 2.

At physiological pH, only one of the two nitrogens in the imidazole ring of the distal histidine of myoglobin may be protonated. Nδ faces the solution, from which it could easily take up a proton. Nϵ faces an oxygen molecule bound to the iron atom in the centre of the haem ring. A proton bound to Nϵ would be ideally placed to form a hydrogen bond (dashed line) to one of the two oxygen atoms of the oxygen molecule. X‐ray crystallography was unable to determine which of the two nitrogens is normally protonated. The picture was produced by using SYBYL (Tripos) using coordinates (2HHD) from the Brookhaven Protein Databank.

Figure 3.

Two‐dimensional projection of the neutron and X‐ray scattering density of the imidazole ring of histidine. (a) Neutron scattering shows the negative scattering density of a hydrogen ion bound to Nϵ. (b) Replacement of the hydrogen ion with a deuterium ion, by soaking the myoglobin crystal in heavy water, results in a region of strong scattering next to Nϵ. (c) A corresponding projection of the X‐ray scattering density. Notice that the hydrogen peaks are hard to distinguish from the background and it is difficult to differentiate between the carbons and nitrogens of the imidazole ring. (d) Ball and stick representation of the structure of the imidazole ring.

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References

Bernal JD, Fankuchen I and Perutz M (1938) An X‐ray study of chymotrypsin and haemoglobin. Nature 141: 523–524.

Frolich A, Gabel F, Jasnin M et al. (2009) From shell to cell: neutron scattering studies of biological water dynamics and coupling to activity. Faraday Discussions 141: 117–130.

Kendrew JC (1950) The crystal structure of horse myoglobin. In: Roughton FJW and Pages JCK (eds) Haemoglobin, pp. 149–160. New York, NY: Interscience Publishers, Inc.

Langan P and Chen JC‐H (2013) Seeing the chemistry in biology with neutron crystallography. Physical Chemistry Chemical Physics 15: 13705–13712.

Moore FM, Willis BT and Hodgkin DC (1967) Structure of a monocarboxylic acid derivative of vitamin B 12. Crystal and molecular structure from neutron diffraction analysis. Nature 214: 130–133.

Phillips SEV and Schoenborn BP (1981) Neutron‐diffraction reveals oxygen‐histidine hydrogen‐bond in oxymyoglobin. Nature 292: 81–82.

Strain HH, Crespi HL and Katz JJ (1959) Choroplast pigments of deuterated green algae. Nature 184: 730–731.

Further Reading

Bacon GE (1975) Neutron Diffraction. Oxford: Clarendon Press.

Fitter J, Gutberlet T and Katsaras J (2006) Neutron Scattering in Biology Techniques and Applications. Berlin, Heidelberg, New York: Springer.

Lakey J (2009) Neutrons for biologists: a beginners guide, or why you should consider using neutrons. Journal of the Royal Society Interface 6: S567–S573.

Schoenborn BP and Knott RB (1996) Neutrons in Biology. New York, NY: Plenum Press.

Willis B and Carlile C (2013) Experimental Neutron Scattering. Oxford: Oxford University Press.

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How to Cite close
Bradshaw, Jeremy P(Sep 2014) Neutron Diffraction. In: eLS. John Wiley & Sons Ltd, Chichester. http://www.els.net [doi: 10.1002/9780470015902.a0003045.pub3]