High-resolution crystallographic studies of native concanavalin A using rapid Laue data collection methods and the introduction of a monochromatic large-angle oscillation technique (LOT)

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Abstract
With synchrotron radiation (SR) high-intensity, polychromatic X-ray beams have become available to protein crystallographers. In Laue diffraction the white beam is used, thus making rapid data collection possible and opening the way for time-resolved experiments. However, Laue patterns suffer from high local densities of spots (leading to spatial overlaps) and consist of up to 12.5% of multiple-order reflections (energy overlaps). Both these problems lead to incomplete datasets and some systematic errors in the corresponding electron density maps. We report here data collection on native concanavalin A and subsequent structure analysis to 1.95 Å resolution. The number of spatial overlaps in the Laue patterns was reduced through the use of a three-dimensional (3D) detector (the so-called toast-rack detector) which records the diffraction pattern at various distances, i.e. in different magnifications, simultaneously. Laue spots consisting of two harmonics were deconvoluted in order to improve further the completeness of the dataset, especially at low resolution. The high resolution (1.95 Å) was realized using a relatively narrow bandpass (± 0.2 Å) centred at 0.7 Å wavelength. The Laue data and, in comparison, a dataset consisting of the Laue data combined with additional low-resolution monochromatic diffractometer data, were used to refine the model structure, certain parts of which served as a test for the data quality and are highlighted. Additionally, a new approach towards fast collection of low-resolution data using a monochromatic synchrotron beam is introduced, which is based on large-angle (e.g. 90°) single exposures and a rotating or oscillating crystal. The method, referred to as LOT (large-angle oscillation technique), may be of interest also for routine data collection, e.g. heavy-atom screening. This technique complements the Laue method for rapid measurement of a unique dataset. A scheme based on a 3D arrangement and/or large areas (e.g. 1 m × 1 m) of image plates is also of interest for high-resolution, LOT data collection from macromolecular crystals, with reduced time overheads for data collection.

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