The only known manganese-sequestering biomolecule in mammals, the immune system protein
calprotectin, is the first structurally characterized example of a naturally-occurring hexahistidine
manganese metal binding site in a metalloprotein. Calprotectin is one of only a handful of manganese-
binding metalloproteins which feature an "all-N" (His4 or His6) coordination environment, in contrast to the
general propensity of Mn2+-binding sites to contain a mixture of N- and O-donor amino acid ligands. A
second example is the cupin protein TM1459, which hosts a His4 manganese site and catalyzes the
oxidative cleavage of alkenes. The research proposed here will develop bioinspired functional and
structure models of these sites using novel polyimidazole chelating ligands. This work is significant
because two important scientific problems will be addressed: the need for selective manganese chelators
which would have applications as potential metal-binding therapeutics or as tools for biomedical
research, and the need for catalytic methods for oxidative alkene cleavage based on earth-abundant and
non-toxic metals. The proposed research is organized into two specific aims: (1) Identify and model the
structural and electronic factors responsible for strong Mn2+ binding in calprotectin. Through a
combination of synthesis, structural characterization, spectroscopy, binding studies, and computation,
novel hexadentate polyimidazole ligands will be used to test bioinspired design principles for selective,
high-affinity manganese chelation. These structures will be tested for antibiotic activity against a
manganese-dependent pathogen, S. pneumoniae, to determine whether the manganese-dependent
adhesion and virulence of this pathogen can be attenuated. (2) Model the polyimidazole-coordinated
manganese and iron centers involved in oxidative alkene cleavage. Manganese and iron complexes of
imidazole-rich chelating ligands related to those developed in Aim 1 will be applied, optimized, and
studied in the context of oxidative double bond cleavage; this bioinspired approach is motivated by the
fact that both His4Mn and His4Fe sites in metalloenzymes are competent in this reaction. The
contributions from this research are expected to be significant because these systems will provide more
faithful biomimetic models of this unusual class of metalloprotein sites with biomedically and
technologically important properties. Additionally, by leveraging collaboration between departments and
directly involving undergraduate students from Mississippi State University (MSU) in carrying out the
proposed work, the research environment at MSU will be enhanced, and highly qualified students will be
exposed to bioinorganic chemistry research relevant to NIH's mission.
