β-sheet-like hydrogen bonds interlock the helical turns of a photoswitchable foldamer to enhance the binding and release of chloride

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© 2014 American Chemical Society. Inspired by halorhodopsins use of photoisomerization to regulate chloride, aryltriazole-based foldamers have been created to "catch and release" chloride ions upon light irradiation of end-appended 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 foldamers 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.

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Journal of Organic Chemistry

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