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Issue 15, 2009
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Temperature dependence of rotational disorder in a non-standard amino acid from X-ray crystallography and molecular dynamics simulation

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Abstract

The X-ray single-crystal structure of methyl 2-aminoisobutyrate hydrochloride (Me-AIB), a non-standard amino acid, is reported at 10, 30, 50, 70 and 100 K. Fourier maps indicate the presence of rotational disorder of the hydrogen atoms of the ester methyl group. To study this effect in detail, high resolution data were collected with synchrotron radiation. The non-spherical molecular electron density was predicted with invariom scattering factors and subtracted from the density obtained from a full multipole refinement. This allows disorder to be distinguished from the molecular electron density at each temperature. The disorder is reduced between 100 K and 30 K, but still detectable even at 10 K. Hence, difference densities can be applied for the purpose of electronic structure validation and have the advantage of an absence of noise over Fourier methods. Ultra-low temperature experiments are foreseen to be useful in reducing such kinds of disorder in ultra-high resolution protein crystallography. Molecular dynamics simulations of Me-AIB at temperatures between 10 and 100 K confirm the temperature dependence of the rotational motion of the methyl group seen experimentally. Modeling disorder in X-ray structure analysis will be an interesting future application of molecular dynamics simulations.

Graphical abstract: Temperature dependence of rotational disorder in a non-standard amino acid from X-ray crystallography and molecular dynamics simulation

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Supplementary files

Article information


Submitted
29 Oct 2008
Accepted
22 Jan 2009
First published
20 Feb 2009

Phys. Chem. Chem. Phys., 2009,11, 2601-2609
Article type
Paper

Temperature dependence of rotational disorder in a non-standard amino acid from X-ray crystallography and molecular dynamics simulation

B. Dittrich, J. E. Warren, F. P. A. Fabbiani, W. Morgenroth and B. Corry, Phys. Chem. Chem. Phys., 2009, 11, 2601
DOI: 10.1039/B819157C

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