Characterization of gadolinium complexes for SAD phasing in macromolecular crystallography: application to CbpF

Seven Gd complexes were used in the preparation of heavy-atom derivatives for solving the structure of choline-binding protein F (CbpF), a 36 kDa surface protein from Streptococcus pneumoniae, by the SAD method. CbpF was used as a model system to analyse the phasing capability of each of the derivatives. Three different aspects have been systematically characterized: the efficacy of cocrystallization versus soaking in the binding of the different Gd complexes, their mode of interaction and a comparative study of SAD phasing using synchrotron radiation and using a rotating-anode generator. This study reveals the striking potential of these complexes for SAD phasing using a laboratory source and further reinforces their relevance for high-throughput macromolecular crystallo­graphy.