Subtle Modification of Imine-linked Helical Receptors to Significantly Alter their Binding Affinities and Selectivities for Chiral Guests

Modulating Helix-Preference of an Axially-Twisted Molecular Scaffold Through Diastereomeric Salt Formation with Tartaric Acid Stereoisomers

Herein, we designed a chiral, axially-twisted molecular scaffold (ATMS) using pyridine-2,6-dicarboxamide (PDC) unit as pivot, chiral trans-cyclohexanediamine (CHDA) residues as linkers, and pyrene residues as fluorescent reporters. R,R-ATMS exclusively adopted M-helicity and produced differential response in UV-vis, fluorescence, and NMR upon addition of tartaric acid (TA) stereoisomers allowing naked-eye detection and enantiomeric content determination. Circular dichroism (CD) profile of R,R-ATMS underwent unique changes during titration with TA stereoisomers - while loss of CD signal at 345 nm was observed with equimolar D-TA and meso-TA, inversion was seen with equimolar L-TA. Temperature increase weakened these interactions to partially recover the original CD signature of R,R-ATMS. 2D NMR studies also indicated the significant structural changes in R,R-ATMS in the solution state upon addition of L-TA. Single crystal X-ray diffraction (SCXRD) studies on the crystals of the R,R-ATMS⊃D-TA salt revealed the interacting partners stacked in arrays and ATMS molecules stabilized by π-π stacking between its PDC and pyrene residues. Contrastingly, tightly-packed supramolecular cages comprised of four molecules each of R,R-ATMS and L-TA were seen in R,R-ATMS⊃L-TA salt, and the ATMS molecules contorted to achieve CH-π interactions between its pyrene residues. These results may have implications in modulating the helicity of topologically-similar larger biomolecules.

Tweezer-type binding cavity formed by the helical folding of a carbazole-pyridine oligomer

We have synthesised a new aromatic foldamer based on the carbazole-pyridine oligomers that adopt helical conformations via dipole-dipole interactions and π-stacking between two ethynyl bond-linked monomers. This foldamer scaffold has been further modified into a synthetic receptor with a tweezer-type binding cavity outside the helical backbone upon folding, in contrast to most aromatic foldamers with internal binding cavities. The tweezer-type cavity is composed of two parallel pyrenyl planes, allowing for the intercalation of a naphthalenediimide guest via π-stacking and CH⋯O interactions, as demonstrated using its ¹H NMR spectra and X-ray crystal structure.