Aromatic Pentaamide Macrocycles as High-Affinity Sulfate Binders and Efficient Transporters

The binding and transport of the highly hydrophilic sulfate ion pose a significant challenge. Receptors capable of both strong sulfate binding and efficient transport are rare. Macrocycles c5, featuring a constrained aromatic amide backbone that convergently locks multiple amide NH and phenyl NH donors, along with five large amide dipoles, create a preorganized, electropositive cavity that binds sulfate in a 1:1 stoichiometry, with affinities (K_a's) from 10⁸ to 10¹³ M^-1 in solvents of low to high polarity. The water-soluble c5c binds sulfate, forming a 4:1 complex in aqueous solution. Macrocycles c5a and c5b with hydrophobic sidechains serve as carriers that mediate efficient sulfate transport across both a bulk chloroform phase and lipid bilayers. With their synthetic tunability, macrocycles c5 can be tailored to media of different properties, yielding effective sulfate binders and carriers with broad applicability.