β?Sheet-like Hydrogen Bonds Interlock the Helical Turns of a Photoswitchable Foldamer To Enhance the Binding and Release of Chloride

摘要

Inspired by halorhodopsin's use of photoisomerization to regulate chloride, aryltriazole-based foldamers have been created to "catch and release" chloride ions upon light irradiation of endappended azobenzenes. The proposed mode of stabilization exploits a β-sheet-like hydrogen-bonding array to cooperatively interlock the ends of a foldamer together with its helical core. We find that the hydrogen-bonding array has a greater influence on stabilizing the helix than the π-stacked seam under the conditions examined (50:50 MeCN/THF). Thus, we show how it is possible to enhance the difference between Cl~? binding and release using light-dependent control over the foldamer's degree of helix stabilization. Making and breaking three π?π contacts with light caused an 8-fold change in chloride affinity (40 300 M~(?1) ? 5000 M~(?1)), five π?π contacts produced a 17-fold change (126 000 M~(?1) ? 7400 M~(?1)), and strategically located hydrogen-bonding units enabled a greater 84-fold differential (970 000 M~(?1) ? 11 600 M~(?1)). The improved performances were attributed to stepwise increases in the preorganization of the binding pocket that catches chloride while leaving the cis states with just one π?π contact relatively unchanged.