Structural biology of the lanthanides—mining rare earths in the Protein Data Bank

With its about 100,000 three-dimensional structures, the Protein Data Bank is a copious source of information: it
contains also some hundreds of structures of macromolecules complexed with lanthanide cations, which are
examined here. These cations, which are found in a wide variety of protein types, were introduced to determine
the structures, by exploiting their anomalous dispersion (in crystallographic studies, where they are also used as
crystallization additives) or the paramagnetic pseudocontact shifts (inNMR analyses). The coordination numbers
in the first coordination sphere are very variable, though they tend to be close to those that are observed in small
molecules or inwater solution. The coordination polyhedra are also quite variable as it can be expected for large
cations. Interestingly, lanthanide cations are frequently observed in packing bridges between symmetry equivalent
molecules in crystals, where they tend to form polynuclear complexes, with up to seven cations bridged by
water/hydroxide ligands.