Abstract
Neutron and X-ray crystallography are complementary to each other. While X-ray scattering is directly proportional to the number of electrons of an atom, neutrons interact with the atomic nuclei themselves. Neutron crystallography therefore provides an excellent alternative in determining the positions of hydrogens in a biological molecule. In particular, since highly polarized hydrogen atoms (H+) do not have electrons, they cannot be observed by X-rays. Neutron crystallography has its own limitations, mainly due to inherent low flux of neutrons sources, and as a consequence, the need for much larger crystals and for different data collection and analysis strategies. These technical challenges can however be overcome to yield crucial structural insights about protonation states in enzyme catalysis, ligand recognition, as well as the presence of unusual hydrogen bonds in proteins.
Highlights
X-ray crystallography has become the workhorse of structural biology, Neutron crystallography has several advantages to offer in the structural analysis of biological molecules
The goal here is not to describe every step in full detail, as these are very similar to X-ray crystallography, but to focus on how crystallographic data treatment pertains to neutron crystallography
The Protein Data Bank (PDB) file is in a plain-text format, which contains the atomic coordinates and holds a variety of extra information regarding the biomolecule itself, and the crystal if the model has been derived from a crystallography experiment
Summary
X-ray crystallography has become the workhorse of structural biology, Neutron crystallography has several advantages to offer in the structural analysis of biological molecules. Less than 5% of deposited models in the Protein Data Bank (PDB) were obtained from crystals diffracting X-rays to (sub-)atomic resolution, and even in the structures with resolution better than 0.5 Å, a third of the expected hydrogens could still not be experimentally identified [2]. It is even able to identify highly polarized H atoms or protons (H+), which cannot be observed by X-rays as they do not have any electrons This neutron specificity arises from the coherent scattering length of the two stable isotopes of hydrogen (1H and 2H ( delineated deuterium, D)) being of similar magnitude to that of other atoms which compose a biological macromolecule (Figure 1). We will describe the challenges that neutron crystallography needs to face, the instrumentation and different crystallographic method used at various sources (reactor versus spallation), the data reduction step and model refinement, and finish with detailed examples of biological questions which can be addressed using neutron crystallography
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