Macromolecular crystallography for f-element complex characterization

Single crystal X-ray diffraction is a technique that measures interatomic distances with atomic resolution. Utilizing this technique for metal complexes featuring lanthanide and actinide elements is complicated by the scarcity and radioactivity of many of the metals of the f-block, as sub-milligram samples are difficult to crystallize for small molecule X-ray diffraction experiments. In this chapter, we present a protocol developed in our group that circumvents these challenges by exploiting macromolecular crystallography, wherein a protein with a large and well-characterized binding calyx is used as a scaffold to crystallize small-molecule metal complexes. Highlighting several examples, we identify the structural and chemical information that can be acquired by this method, and delineate the benefits of directing crystal growth with proteins, such as decreasing the amount of metal used to the sub-microgram scale. Moreover, since protein recognition depends on the nature of the metal-chelator bonds, subtle effects in the lanthanide and actinide coordination chemistry, such as metal-ligand covalency, can be qualitatively assessed.