Xin Wu | Nature Chemistry: A charge-neutral organic cage selectively binds strongly hydrated sulfate anions in water

Abstract

In biological systems, enzymes and transport proteins can bind anions in aqueous media solely by forming hydrogen bonds with charge-neutral motifs. Reproducing this functionality in synthetic systems presents challenges and incurs high costs, particularly when targeting strongly hydrated anions such as sulfate. Here we report a [2.2.2]urea cryptand (cage), synthesized in one pot, that selectively binds sulfate in a mixture of dimethyl sulfoxide and water and in water with affinities in the micromolar to millimolar range. The neutral cage bearing six urea groups donates 12 strong hydrogen bonds to encapsulate a sulfate anion, showing favourable enthalpy even in pure water. Sulfate binding can be further enhanced by using micelles to provide a low-polarity microenvironment. The cage finds utility in analysing divalent anions in water and beverage samples or in removing sulfate. The work demonstrates the achievability of robust and selective anion binding in water with minimal synthetic efforts, by using neutral NH hydrogen bonds akin to those found in biology.

Caught in a trap

Sulfate-binding proteins — which capture sulfate from water using hydrogen bonds from charge-neutral motifs — serve as key inspiration in anion receptor chemistry, but synthetic systems showing similar selectivity have so far been elusive. Xin Wu, Evelyne Deplazes and colleagues have now made a neutral molecular cage that binds sulfate in water using 12 hydrogen atoms, as depicted artistically on the cover. The background shows an empty cage and an unbound sulfate.

Link: https://www.nature.com/articles/s41557-024-01457-5