Self-Assembly of Chiral Metal-Organic Cages at the Presence of Chiral Counterions: Chirality Effects on the Intermolecular Interactions

One of the most important questions about the chirality of biological molecules is their effect on the intermolecular interactions specially during supramolecular formation leading to homochiral structures. In this work, positively charged alanine-based D- and L-Pd₁₂Ala₂₄ metal organic cages (MOCs) self-assemble into single-layered hollow spherical blackberry-type supramolecular structures by adding extra nitrates through counterion-mediated attraction. Moreover, it was observed that enantiomers of small chiral counterions, although possessing similar chemical and physical properties, have a hugely different effect on the inter-cage electrostatic interaction. While D-counterion does not show any significant effect on the interaction between D-Pd₁₂Ala₂₄ MOCs and consequently on their self-assembly behavior, L-counterions notably reduce the self-assembly rate, size of the blackberry structures, and concentration of the assemblies by having a higher binding strength to the MOCs and inhibiting the nitrate counterions, which trigger the self-assembly process, to bind to the cages. The same story can be observed for L-Pd₁₂Ala₂₄ and D-counterions. This work provides evidence on homochirality phenomena observed in biological supramolecules.