Influence of divalent cations in the protein crystallization process assisted by Lanthanide-based additives.

The use of lanthanide complexes as powerful auxiliaries for biocrystallography prompted us to systematically analyze the influence of the commercial crystallization kit composition on the efficiency of two lanthanide additives: [Eu(DPA)3]3- and Tb-Xo4. This study reveals that the tris-dipicolinate complex presents a lower chemical stability and a strong tendency to false positives which are detrimental for its use in high-throughput robotized crystallization platform. On the other hand, the Tb-Xo4 is perfectly soluble in the crystallization media, stable in the presence of alkaline-earth dications and slowly decomposed (within days) by trans-metalation with transition metals. This article also highlight a potential synergy of interactions between the Tb-Xo4 and components of the crystallization mixtures leading to the formation of complex adducts like {AdkA/Tb-Xo4/Mg2+/glycerol} in the protein binding sites.