Protein Crystallography by Alexander Wlodawer Zbigniew Dauter & Mariusz Jaskolski

Author:Alexander Wlodawer, Zbigniew Dauter & Mariusz Jaskolski
Language: eng
Format: epub
Publisher: Springer New York, New York, NY

Partiality can be thought of as a generalization of the problem of determining which reflections are excited in a given diffraction snapshot. In an SX experiment, without the advantage of surrounding frames in a rotation series to help, this determination must be performed using only the information from the snapshot itself. It might seem helpful to assign indices to all the visible peaks in the diffraction pattern and integrate only those, but this would neglect weak reflections. To accurately measure the intensity of a reflection, all intensity measurements from different frames should be combined, some below and some even slightly above the true value, according to the limited precision of an individual measurement. For a very weak reflection, some or perhaps all of these measurements may not correspond to obviously identifiable peaks in the diffraction patterns, and some of the estimations may even be negative after subtracting the background . Consider a hypothetical experiment in which indexing indicates that we made five measurements of a certain reflection in different snapshots and found that only one of the measurements indicated a high intensity, the rest giving very low values below their estimated error. Did we measure a strong reflection which we incorrectly determined to be excited in four of the snapshots, or did we in fact determine the excitation of the reflection correctly and measured a weak reflection with one outlier due to some other influence? By correcting for partialities, we extend the simple yes/no decision about the excitation of each reflection to a smoothly varying factor describing how excited the reflection is, in turn describing how precisely the reflection can give information about the underlying structure factor modulus.

Partiality is also a consideration in rotation experiments performed using an XFEL. Hirata et al. [10] investigated the effect of the size of the rotation step between snapshots in such an experiment, and found the optimal step size to be one third of the mosaicity of the crystal. The snapshot diffraction patterns were processed using a standard program for rotation data analysis, which was not aware that the patterns were snapshots rather than rotation wedges. Nevertheless, the final data quality compared favourably with a reference experiment using a synchrotron X-ray source.

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