Determination of the Molecular Structures of Ferric Enterobactin and Ferric Enantioenterobactin Using Racemic Crystallography

Enterobactin
is a secondary metabolite produced by Enterobacteriaceae
for acquiring iron, an essential metal nutrient. The biosynthesis
and utilization of enterobactin permits many Gram-negative bacteria
to thrive in environments where low soluble iron concentrations would
otherwise preclude survival. Despite extensive work carried out on
this celebrated molecule since its discovery over 40 years ago, the
ferric enterobactin complex has eluded crystallographic structural
characterization. We report the successful growth of single crystals
containing ferric enterobactin using racemic crystallization, a method
that involves cocrystallization of a chiral molecule with its mirror
image. The structures of ferric enterobactin and ferric enantioenterobactin
obtained in this work provide a definitive assignment of the stereochemistry
at the metal center and reveal secondary coordination sphere interactions.
The structures were employed in computational investigations of the
interactions of these complexes with two enterobactin-binding proteins,
which illuminate the influence of metal-centered chirality on these
interactions. This work highlights the utility of small-molecule racemic
crystallography for obtaining elusive structures of coordination complexes.