Synthetic chloride transporters containing two urea groups linked through a diazobenzene spacer have been prepared and the trans-to-cis isomerization by light stimulation results in dramatic changes in the chloride transport activities across lipid and cell membranes.
Binding of α-amino carboxylates to a helically twisted imine macrocycle based on the indolocarbazole scaffold gives rise to characteristic circular dichroism spectra, and the patterns of the Cotton effects are consistent with the absolute configuration of α-amino carboxylates.
A series of indolocarbazole-pyridine (IP) oligomers were prepared that fold into helical conformations, and their folding features in solution and in the solid state were revealed. Helical folding of these IP foldamers is induced by dipolar interactions through the ethynyl bond and π-stacking between two repeating units. Upon helical folding, (1)H NMR signals of aromatic protons were significantly shifted upfield by Δδ = 0.5-2.2 ppm. In addition, hypochromic shifts and fluorescence quenching were observed in the absorption and emission spectra. X-ray crystal structures clearly demonstrated that IP foldamers folded to helical structures with cylindrical internal cavities wherein 3 or 5 water molecules were occupied by hydrogen-bonding interactions in a 1-D array, reminiscent of transmembrane water channels, called aquaporins.
Template‐directed synthesis and dynamic covalent chemistry were implemented to achieve quantitative one‐pot syntheses of homochiral helical cavities inside aromatic foldamers. One‐handed helical receptors P‐1, M‐1, P‐2 and M‐2 were assembled from their precursors in the presence of appropriate templates (d‐ and l‐tartaric acid, and d‐ and l‐sorbitol, respectively) via three sequential steps in one pot: imine‐linked chain elongation, template‐induced folding and [4+2] cycloaddition between helical turns. These helical receptors were proven to enantioselectively bind chiral guests used as the templates, and the differences between the association constants of enantiomeric guests were up to more than two orders of magnitude. The structures and binding modes of the receptors were fully characterized by single‐crystal X‐ray crystallography and 1H NMR spectroscopy.
As a synthetic model for intra‐protein interactions that reinforce binding affinities between proteins and ligands, the energetic interplay of binding and folding was investigated using foldamer‐based receptors capable of adopting helical structures. The receptors were designed to have identical hydrogen‐bonding sites for anion binding but different aryl appendages that simply provide additional π‐stacking within the helical backbones without direct interactions with the bound anions. In particular, the presence of electron‐deficient aryl appendages led to dramatic enhancements in the association constant between the receptor and chloride or nitrate ions, by up to three orders of magnitude. Extended stacking within the receptor contributes to the stabilization of the entire folding structure of complexes, thereby enhancing binding affinities.
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