In spite of the fact that structure solving methods are constantly improving, the biggest challenge of protein crystallography remains the production of well diffracting single protein crystals. Full understanding the environmental factors that influence crystal packing would be an enormous task, therefore crystallographers are still forced to work "blindly" trying as many crystallizing conditions and mutations, designed to improve crystal packing, in the sequence of the target protein as possible. Numerous times the random attempts simply fail even when using crystallization robots or recent techniques to determine the optimal mutations. As an alternative option in these cases, crystallization chaperones can be used. These proteins have a unique property, namely they easily form protein crystals, which can be exploited by using them as a heterologous fusion partner to promote crystal contact formation. Today, the most frequently used crystallization chaperone is the maltose-binding protein (MBP) and crystallographers are in need of other options. Our previous results showed the outstanding crystallization properties of a non-EF hand calcium-binding protein annexin A2 (ANXA2). Here, we compared ANXA2 with the wild type MBP and found that ANXA2 is just as good, if not a better crystallization chaperone. Using ANXA2 for this purpose, we were able to solve the atomic resolution structure of a challenging crystallization target, the transactivation domain (TAD) of p53 in complex with S100A4, an EF hand calcium-binding protein associated with metastatic tumors. The full-length TAD forms an asymmetric fuzzy complex with S100A4 and could interfere with its function.

A regio- and diastereoselective 1,3-dipolar cycloaddition of 2H-azirines with azomethine ylides generated in situ from isatins and α-amino acids has been elaborated, affording an unprecedented aziridine-fused spiro­[imidazolidine-4,3′-oxindole] framework. This one-pot three-component reaction tolerates a wide range of substrates and enables the construction of highly diverse 1,3-diazaspiro­[bicyclo[3.1.0]­hexane]­oxindoles in isolated yields up to 81% under mild conditions.

Enzymes of the prolyl oligopeptidase family (S9 family) recognize their substrates not only by the specificity motif to be cleaved but also by size - they hydrolyze oligopeptides smaller than 30 amino acids. They belong to the serine-protease family, but differ from classical serine-proteases in size (80 kDa), structure (two domains) and regulation system (size selection of substrates). This group of enzymes is an important target for drug design as they are linked to amnesia, schizophrenia, type 2 diabetes, trypanosomiasis, periodontitis and cell growth. By comparing the structure of various members of the family we show that the most important features contributing to selectivity and efficiency are: (i) whether the interactions weaving the two domains together play a role in stabilizing the catalytic triad and thus their absence may provide for its deactivation: these oligopeptidases can screen their substrates by opening up, and (ii) whether the interaction-prone β-edge of the hydrolase domain is accessible and thus can guide a multimerization process that creates shielded entrance or intricate inner channels for the size-based selection of substrates. These cornerstones can be used to estimate the multimeric state and selection strategy of yet undetermined structures.

In macromolecular crystallography, a great deal of effort has been invested in understanding radiation‐damage progression. While the sensitivity of protein crystals has been well characterized, crystals of DNA and of DNA–protein complexes have not thus far been studied as thoroughly. Here, a systematic investigation of radiation damage to a crystal of a DNA 16‐mer diffracting to 1.8 Å resolution and held at 100 K, up to an absorbed dose of 45 MGy, is reported. The RIDL (Radiation‐Induced Density Loss) automated computational tool was used for electron‐density analysis. Both the global and specific damage to the DNA crystal as a function of dose were monitored, following careful calibration of the X‐ray flux and beam profile. The DNA crystal was found to be fairly radiation insensitive to both global and specific damage, with half of the initial diffraction intensity being lost at an absorbed average diffraction‐weighted dose, D1/2, of 19 MGy, compared with 9 MGy for chicken egg‐white lysozyme crystals under the same beam conditions but at the higher resolution of 1.4 Å. The coefficient of sensitivity of the DNA crystal was 0.014 Å2 MGy−1, which is similar to that observed for proteins. These results imply that the significantly greater radiation hardness of DNA and RNA compared with protein observed in a DNA–protein complex and an RNA–protein complex could be due to scavenging action by the protein, thereby protecting the DNA and RNA in these studies. In terms of specific damage, the regions of DNA that were found to be sensitive were those associated with some of the bound calcium ions sequestered from the crystallization buffer. In contrast, moieties farther from these sites showed only small changes even at higher doses.

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Pyranuronic β‐sugar amino acids (β‐SAAs) are biocompatible and tuneable building blocks of foldamers and chimera‐peptides. The scalable and economical total synthesis of two building blocks is described here. These C‐4 epimers, Fmoc‐GlcAPU(Me)‐OH (7) and Fmoc‐GalAPU(Me)‐OH (8), which are suitable for solid phase peptide synthesis, were prepared via a common oxime intermediate 16. The new synthesis uses nine consecutive steps, starting from methyl α‐d‐glucopyranoside (6). The synthesis is fine‐tuned, optimized, and ready for large‐scale and cost‐efficient production.

The single-crystal structures of calcium d-gluconate and calcium -d-isosaccharinate have been determined using X-ray diffraction at 100 K. Surprisingly, given its significance in industrial and medical applications, the structure of calcium d-gluconate has not previously been reported. Unexpectedly, the gluconate crystal structure comprises coordination polymers. Unusually, the calcium coordination number is nine. Adjacent metal centres are linked by three -oxo bridges, with a metal-metal separation of 3.7312 (2) Å . One of the gluconate ligands contradicts a suggestion from 1974 that a straight chain conformation is associated with an intramolecular hydrogen bond. This ligand binds to three adjacent metal centres. The use of synchrotron radiation provided an improved crystal structure with respect to that previously reported for the isosaccharinate complex, allowing the location of the hydroxy hydrogen sites to be elucidated. In contrast to the gluconate structure, there are no -oxo bridges in the isosaccharinate coordination polymer and the isosaccharinate bridging coordination is such that the distance between adjacent metal centres, each of which is eight-coordinate, is 6.7573 (4) Å . Complementing the crystal structure determinations, modelling studies of the geometries and coordination modes for the aqueous [CaGluc] + and [CaIsa] + complexes are presented and discussed. research papers Acta Cryst. (2018). B74, 598-609 V. Bugris et al. Calcium complexes relevant to problematic waste disposal 599

Calmodulin (CaM) is a highly conserved eukaryotic Ca2+ sensor protein that is able to bind a large variety of target sequences without a defined consensus sequence. The recognition of this diverse target set allows CaM to take part in the regulation of several vital cell functions. To fully understand the structural basis of the regulation functions of CaM, the investigation of complexes of CaM and its targets is essential. In this minireview we give an outline of the different types of CaM - peptide complexes with 3D structure determined, also providing an overview of recently determined structures. We discuss factors defining the orientations of peptides within the complexes, as well as roles of anchoring residues. The emphasis is on complexes where multiple binding modes were found.