Chasing nature’s anion binders

Solute binding proteins can capture strongly hydrated substrates from water with high affinity. Notable examples are the sulfate binding proteins (SBPs), which use neutral NH and OH groups to stabilize highly charged sulfate anions without salt-bridge formation. Synthetic receptors are yet to achieve comparable capabilities solely by hydrogen bonds or other ion-dipole interactions. Here we report charge-neutral urea cages which have strong affinities for CrO42–, SO42– and Cl– up to micromolar and sub-micromolar levels in water, with tunable anion selectivity based on size match. This is despite the fact that these urea cages are much smaller than proteins and have their anion binding sites exposed to water, demonstrating synthetic receptors’ hydrogen bonding potency that emulates nature’s anion binders.