Hierarchical Self-Assembly of Metal-Organic Cages: Exploring Chiral Recognition and Homochirality

Hydrophilic macroions (1–5 nm in size) with moderate charges exhibit unique solution behaviors unlike those of simple ions or colloidal particles. In dilute aqueous solutions, the size disparity between macroions and small counterions leads to moderate counterion association, which in turn triggers counterion-mediated attraction. This attraction among like-charged macroions results in the formation of stable, hollow, single-layered blackberry type structures, which resemble virus capsids in their spherical organization. Such self-recognition is a typical feature of biomacromolecules but is unexpected for common macroions.

Structurally defined metal-organic cages (MOCs) serve as examples of these macroions. Incorporating chiral amino acids into MOCs creates enzyme-like pockets that facilitate chiral recognition, leading to homochiral blackberry assemblies. We found that chiral recognition is only achieved when the amino acid counterions are as hydrophobic or less so than those within the MOCs. Furthermore, positioning amino acid ligands either outward or inward in MOCs reveals differing chiral recognition behaviors, highlighting the importance of spatial arrangement between chiral centers in MOCs and counterions. This work provides insights into self-recognition and homochirality, bridging concepts between synthetic and biological systems.