Date Published: February 01, 2019
Publisher: International Union of Crystallography
Author(s): Michele Cianci, Max Nanao, Thomas R. Schneider.
A long-wavelength mesh data collection using a size-tailored microbeam from concanavalin A microcrystals with linear dimensions of less than 20 µm allowed experimental phase determination using the anomalous signal from naturally occurring Mn2+ and Ca2+ ions.
De novo determination of macromolecular structures requires the accurate measurement of structure factors and retrieval of experimental phases from the crystals of the given specimen. When a model with significant structure similarities is available, phases can be retrieved using the molecular-replacement (MR) method. Otherwise, the phases must be determined experimentally. One experimental phasing method that is gaining popularity is the use of the anomalous signal from naturally occurring anomalous (native SAD) or from ad hoc incorporated anomalous scatterers (International Tables for Crystallography, 2012 ▸).
The linear dimensions of macromolecular crystals that are considered to be usable for X-ray data collection are continally becoming smaller and smaller, potentially reaching the point where crystals will be too small for optical centring and for individual data collection, but large enough to be detectable with low-dose X-ray centering and partial data-set collection. The availability of a high-intensity and well collimated beam, as on the P13 beamline (Cianci et al., 2017 ▸), permits tailoring of the beam size to the sample size thanks to variable apertures, thus minimizing the scattering background.