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Sergio Morelhao1 2 Cláudio Remédios3 Stefan Kycia2

1, University of Sao Paulo, Sao Paulo, SP, Brazil
2, University of Guelph, Guelph, Ontario, Canada
3, Universidade Federal do Pará, Belém, PA, Brazil

Continuing improvement in X-ray sources, softwares and instrumental automation during decades are the major reasons for crystallographic studies been conducted by an increasing number of non-experts in nowadays. However, besides diffracted intensities, the scattering length of X-rays also allows experimental measurements of structure factor phases. Physical phase measurement via dynamical diffraction experiments is the finest tool for probing structural features that are not accessible by other methods. For instance, the most recent example of its potential in crystallography is the detection of electron charges in hydrogen bonds responsible for intermolecular forces between amino acid molecules [1]. Although a lot has been developed lately in terms of how to carry out phase measurements, this method is still at similar situation of the standard X-ray methods decades ago where each successful application demands a lot of effort.
In this work, we carried out phase measurements in amino acid crystals using an in-house X-ray generator, showing that even with a low flux beam hydrogen bonds in biological molecules can be investigated. Synchrotron data is also shown for sake of comparison. One of the greatest challenging to use this method in single crystals of small dimensions (<1mm) is how to perform data collection with the sample spinning around an specific crystallographic direction. We derive an in-situ alignment procedure based on the crystal rotating method commonly used in protein crystallography. It allows a final alignment between the crystallographic direction and the spinning axis better than 0.01 degrees, which is necessary for high quality data aiming absolute refinements of crystal structures [2]. Furthermore, data analysis procedures for retrieving phase values are detailed and compared with theoretical values from model structure for asparagine, glycine, and histidine single crystals.

[1] S. L. Morelhão, C. M. R. Remédios, G. A. Calligaris, G. Nisbet. J. Appl. Cryst. 50, 689 (2017).
[2] S. L. Morelhão, Z. G. Amirkhanyan, C. M. R. Remédios. Acta Cryst. A71, 291 (2015).

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