An excellent chiral symmetry-breaking spontaneous enantiomeric resolution phenomenon, denoted preferential enrichment, was observed on recrystallization of the 1:1 cocrystal of dl-arginine and fumaric acid, which is classified as a racemic compound crystal with a high eutectic ee value (>95 %), under non-equilibrium crystallization conditions. On the basis of temperature-controlled video microscopy and in situ time-resolved solid-state (13) C NMR spectroscopic studies on the crystallization process, a new mechanism of phase transition that can induce preferential enrichment is proposed.

In this study, we present the X-ray structure of the NAC:DNA complex to 4.4 Å. The structure was solved with molecular replacement (MR) using as search models the apo-structure of NAC (1UT4) and a B-DNA model of the oligonucleotide (X.-J. Lu and W. K. Olsen (2003) Nucleic Acids Res 31(17):5108-5121). Refinement proved complicated due to the low resolution. The best strategy turned out to include NCS restraints on all atoms, reference-model restraints on protein atoms, and TLS. Phase restraints were available but did not improve refinement. Neither did simulated annealing.The exact position of the individual DNA bases along the main DNA axis could not be determined by MR alone. Instead, the DNA sequence was assigned based on a uranyl photoprobing assay. The resulting model could be refined to R work =24.1% and R free =34.4%.The structure provides for the first time experimental evidence for the speculated evolutionary relationship between the plant-specific NAC proteins and other transcription factors with a surprising phylogenetic breath. The structure shows how the NAC domain inserts the edge of its core beta-sheet in the major groove (figure), while leaving the DNA largely un-distorted. Recognition of DNA with the edge of a beta-sheet is also believed to be a feature of the WRKY family of transcription factors (Rushton et al. (2010) Trends Plant Sci 15(5):247-258), which are found in early eukaryotes and plants. Further, the animal GCM transcription factors use the same binding motif (Cohen et al. (2003) EMBO J 22(8):1835EMBO J 22(8): .1845). In addition, these three transcription factors share a core beta-sheet with a very similar topology.The presented NAC:DNA complex structure provides a framework for studying the effects of single amino acids as well as structural features on DNA binding affinity and specificity. Further, we have evidence of limited flexibility of the NAC dimer arrangement, which could explain the limited tolerance in NAC binding site spacing that has been reported (Olsen et al. (2005) The investment in modern equipment and the development of highly automated beamline control software on the public MX-beamlines at the ESRF now allows standard X-ray diffraction experiments, even multiple anomalous diffraction (MAD), to be easily and robustly performed. However the success of X-ray diffraction experiments is still dependent on the quality of the crystals obtained. Most modern structural biology projects have become ever more complex and their success is now often dependent on a combination of low resolution techniques such as EM, X-ray small angle scattering and macromolecular crystallography. Many large and complex macromolecular assemblies often fail to crystallise or at best form few poorly diffracting crystals that are radiation sensitive. Such projects require newly developed equipment and a much more careful approach for data collection [1, 2]. In Grenoble we are developing improved instruments and methods for optimised low resolution data collection possibilities. Here I wil...