Back-exchange of deuterium in neutron crystallography: characterization by IR spectroscopy.

Ai Woon Yee,V Trevor Forsyth,Edward Mitchell,Martine Moulin,Michael Haertlein,Matthew P Blakeley

doi:10.1107/s1600576717003624

Abstract

The application of IR spectroscopy to the characterization and quality control of samples used in neutron crystallography is described. While neutron crystallography is a growing field, the limited availability of neutron beamtime means that there may be a delay between crystallogenesis and data collection. Since essentially all neutron crystallographic work is carried out using D2O-based solvent buffers, a particular concern for these experiments is the possibility of H2O back-exchange across reservoir or capillary sealants. This may limit the quality of neutron scattering length density maps and of the associated analysis. Given the expense of central facility beamtime and the effort that goes into the production of suitably sized (usually perdeuterated) crystals, a systematic method of exploiting IR spectroscopy for the analysis of back-exchange phenomena in the reservoirs used for crystal growth is valuable. Examples are given in which the characterization of D2O/H2O back-exchange in transthyretin crystals is described.

Highlights

The use of neutron crystallography to study biological macromolecules has been expanding in recent years (Blakeley et al, 2015)
The aim of the Fourier transform IR (FT–IR) measurements was to provide information regarding the ratio of protium and deuterium atoms present in samples used for neutron crystallographic experiments
This paper demonstrates that FT–IR spectroscopy can be used in a routine way to assess back-exchange during sample preparation and both before and after data collection

Summary

Introduction

The use of neutron crystallography to study biological macromolecules has been expanding in recent years (Blakeley et al, 2015). The advantage of using neutrons is that the coherent scattering lengths for protium (1H) and deuterium (2H) are of a similar magnitude to those of other common elements of a macromolecule (e.g. C, N, O and S) This means that, in contrast with the situation for X-ray diffraction, they have clear visibility in neutron crystallographic studies. The neutron coherent scattering length of protium carries a negative sign, which can cause cancellation effects in neutron scattering length density maps (hereafter referred to as neutron maps) (Fisher et al, 2014) Another important point is that protium, which accounts for about half of the atoms in biological macromolecules, has a very large incoherent scattering cross section (80.27 barn; 1 barn = 100 fm2), arising from the two spin states of the atom.

Methods

Results

Conclusion