Unraveling Chiral Selection in the Self-assembly of Chiral Fullerene Macroions: Effects of Small Chiral Components Including Counterions, Co-ions, or Neutral Molecules

Side Group of Hydrophobic Amino Acids Controls Chiral Discrimination among Chiral Counterions and Metal-Organic Cages

The self-assembly of chiral Pd₁₂L₂₄ metal-organic cages (MOCs) based on hydrophobic amino acids, including alanine (Ala), valine (Val), and leucine (Leu), into single-layered hollow spherical blackberry-type structures is triggered by nitrates through counterion-mediated attraction. In addition to nitrates, anionic N-(tert-butoxycarbonyl) (Boc)-protected Ala, Val, and Leu were used as chiral counterions during the self-assembly of d-MOCs. Previously, we showed that l-Ala suppresses the self-assembly process of d-Pd₁₂Ala₂₄ but has no effect on l-Pd₁₂Ala₂₄, i.e., chiral discrimination. Here, we indicate when the amino acid used as the chiral counterion has a bulkier side group than the amino acid in the MOC structure, no chiral discrimination exists; otherwise, chiral discrimination exists. For example, Ala can induce chiral discrimination in all chiral MOCs, whereas Leu can induce chiral discrimination only in Pd₁₂Leu₂₄. Moreover, chiral anionic d- and l-alanine-based surfactants have no chiral discrimination, indicating that bulkier chiral counterions with more hydropohobic side groups can erase chiral discrimination.

Strong Enantiomeric Preference on the Macroion-Counterion Interaction Induced by Weakly Associated Chiral Counterions

The role of chiral counterions on the attraction and self-assembly of chiral Pd₁₂L₂₄ metal organic cages (MOCs) with NO₃^- being the original counterion is studied by laser light scattering and isothermal titration calorimetry. Nitrates can trigger the self-assembly of macrocationic Pd₁₂L₂₄ into hollow spherical blackberry-type supramolecular structures via counterion-mediated attraction. Although chiral counteranions, such as N-(tert-butoxycarbonyl)-alanine (Boc-Ala), have weaker interaction with the MOCs compared to NO₃^-, they can induce different assembly behaviors between two enantiomeric MOCs by inhibiting the MOC-nitrate binding and weakening the interaction between them. The d-counterions are capable of selectively suppressing and slowing down the assembly of l-MOCs and also considerably decreasing their assembly size due to the much weaker MOC-nitrate interaction. The same scenario is observed for l-counterions when interacting with the d-MOCs. This study unveils the role of weakly associated chiral counterions on the central chiral macroions, especially their supramolecular structure formation, and provides additional evidence on the mechanism of the homochirality phenomenon.