Abstract
The structure of the primary amino acid L-leucine has been determined for the first time by neutron diffraction. This was made possible by the use of modern neutron Laue diffraction to overcome the previously prohibitive effects of crystal size and quality. The packing of the structure into hydrophobic and hydrophilic layers is explained by the intermolecular interaction energies calculated using the PIXEL method. Variable-temperature data collections confirmed the absence of phase transitions between 120 and 300 K in the single-crystal form.
Highlights
The advantages of neutron diffraction for providing accurate geometric parameters for amino acids and other molecular materials are well known, and include the strong and contrasting scattering lengths of hydrogen and deuterium, and the absence of form-factor fall off with scattering angle (Niimura & Bau, 2008, McIntyre, 2014, Görbitz, 2015)
Accurate geometric parameters for hydrogen atoms are especially important as structures derived from X-ray studies suffer from sever systematic errors as a result of aspherical electron density distributions about covalently-bonded hydrogen
The structure of the natural amino acid L-leucine has been determined by neutron diffraction for the first time at temperatures of 300 and 120 K
Summary
The advantages of neutron diffraction for providing accurate geometric parameters for amino acids and other molecular materials are well known, and include the strong and contrasting scattering lengths of hydrogen and deuterium, and the absence of form-factor fall off with scattering angle (Niimura & Bau, 2008, McIntyre, 2014, Görbitz, 2015). Amino acid structural parameters derived from neutron diffraction have found extensive application as constraints and restraints in macromolecular refinements, and many of the entries above are still the preferred standards today and remain heavily cited in the literature. These publications have a mean number of citations of 83 overall, and 20 in the last five years. We report the neutron-diffraction-derived structures of L-leucine at 120 K and room temperature as determined using the KOALA Laue diffractometer at ANSTO. Unit-cell dimensions for the room temperature and 120 K data-sets were taken from corresponding X-ray diffraction studies (Coll et al, 1986, Görbitz & Dalhus, 1996b). Electrostatic potentials were mapped onto these surfaces over the range −0.173 to +0.286 au
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