Metal-organic frameworks (MOFs), which are extended networks of coordinating ligands and metal ions, have proven to be attractive materials for a variety of applications, from gas sorption or separation to catalysis. The synthesis of extant MOFs has been constricted by the commonly used rigid, conjugated aromatic ligands, resulting in tetrahedral, cubic, or octahedral networks. The biomolecule toolbox offers researchers vast structural and functional diversity, coupled with biocompatibility. Peptides are of particular interest, due to their chiral nature and facile synthesis; however, the asymmetry and flexibility of shorter peptides renders the formation of peptide-containing crystalline MOFs problematic.Helma Wennemers and co-workers have reported a truly elegant MOF architecture utilizing chiral, helical oligoproline peptides, or "molecular rulers", as novel ligands (DOI: 10.1021/jacs.0c11793). Modifying the C- and N-termini of the peptides with aromatic and aliphatic carboxylates, respectively, facilitates metal ion coordination, ultimately leading to the formation of novel MOF structures. These brilliant structures consist of stacked pleated peptide sheets containing strings of alternating metal ions. Contrary to traditional MOFs, further analysis of the resulting crystal structure revealed that the peptide ligands serve multiple functions in the structure, coordinating with the metal ion as well as interacting with each other through a unique network of weak, non-covalent interactions. The dual functionality of these biocompatible peptidic ligands with defined secondary structures paves the way for a variety of novel ligand geometries and unveils myriad opportunities to further expand the realm of MOFs.
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