Nowadays, most structures are solved by the molecular replacement (MR) method, which estimates phases from structures of homologous proteins. Nonetheless, it is possible that no structural homolog exists for a protein, or that no homolog is structurally close enough to enable MR phasing, requiring de novo phasing of the diffraction data. A variety of experimental-phasing approaches exist, all of which rely on the perturbation of diffracted intensities. Nowadays, the crystallographers’ favourite is single-wavelength anomalous diffusion (SAD), whereby introduction of an anomalous scatterering in a protein crystal, combined with data collection at a specific wavelength, breaks the centrosymmetric diffraction, enabling phasing based on anomalous differences. The latter are weak, explaining that accurately measured and processed data are required to enable phasing by this technique.
POLYVALAN Xo4 products are unique lanthanide complexes that facilitate crystal structure determination thanks to their bridging role between vicinal protein chains (reinforcement of crystal contacts), and their exceptional anomalous properties. They are therefore straightforward phasing agents overcoming the tedious and time-consuming work of existing solutions: seleno-methionine labelling or heavy-atom incorporation.