Single‐wavelength phasing strategy for quasi‐racemic protein crystal diffraction data
Racemic protein crystallography offers two key features: an increased probability of crystallization and the potential advantage of phasing centric diffraction data. In this study, a phasing strategy is developed for the scenario in which a crystal is grown from a mixture in which anomalous scattering atoms have been incorporated into only one enantiomeric form of the protein molecule in an otherwise racemic mixture. The structure of a protein crystallized in such a quasi‐racemic form has been determined in previous work [Pentelute et al. (2008), J. Am. Chem. Soc. 130, 9695–9701] using the multiwavelength anomalous dispersion (MAD) method. Here, it is shown that although the phases from such a crystal are not strictly centric, their approximate centricity provides a powerful way to break the phase ambiguity that ordinarily arises when using the single‐wavelength anomalous dispersion (SAD) method. It is shown that good phases and electron‐density maps can be obtained from a quasi‐racemic protein crystal based on single‐wavelength data. A prerequisite problem of how to establish the origin of the anomalous scattering substructure relative to the center of pseudo‐inversion is also addressed.
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Document Type: Research Article
Publication date: 2012-01-01