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Protein crystals can be incommensurately modulated

J. Appl. Cryst. (2008). 41, 600–605 (

[Incommensurately modulated protein crystal] Incommensurately modulated PA crystal with main reflections indexed and predicted (blue circles) with space group P222, a = 38.1, b = 72.3, c = 185.7 Å and showing satellite reflections in one modulation direction indexed as q = 0.29b*.
Modulated crystals result in diffraction patterns consisting of main reflections surrounded by satellite reflections. These patterns are typically encountered in small-molecule structures at the interface of a phase transition. If the modulation is incommensurate then the satellite reflections cannot be predicted with a supercell. In these cases a superspace indexing approach is used to index the reflections. We report a successful superspace indexing of an incommensurately modulated crystal of profilin:actin. In this case the modulation is induced with a pH change.
Jeffrey J. Lovelace, Cameron R. Murphy, Lee Daniels, Kartik Narayan, Clarence E. Schutt, Uno Lindberg, Christer Svensson and Gloria E.O. Borgstahl