Abstract
The design of artificial oxyanion receptors with switchable ion preference is a challenging goal in host–guest chemistry. We here report on molecularly imprinted polymers (MIPs) with an external phospho-sulpho switch driven by small molecule modifiers. The polymers were prepared by hydrogen bond-mediated imprinting of the mono- or dianions of phenyl phosphonic acid (PPA), phenyl sulfonic acid (PSA), and benzoic acid (BA) using N-3,5-bis-(trifluoromethyl)-phenyl-Ń-4-vinylphenyl urea (1) as the functional host monomer. The interaction mode between the functional monomer and the monoanions was elucidated by 1H NMR titrations and 1H–1H NMR NOESY supported by molecular dynamic simulation, which confirmed the presence of high-order complexes. PPA imprinted polymers bound PPA with an equilibrium constant Keq = 1.8 × 105 M–1 in acetonitrile (0.1% 1,2,2,6,6-pentamethylpiperidine) and inorganic HPO42– and SO42– with Keq = 2.9 × 103 M–1 and 4.5 × 103 M–1, respectively, in aqueous buffer. Moreover, the chromatographic retentivity of phosphonate versus sulfonate was shown to be completely switched on this polymer when changing from a basic to an acidic modifier. Mechanistic insights into this system were obtained from kinetic investigations and DSC-, MALDI-TOF-MS-, 1H NMR-studies of linear polymers prepared in the presence of template. The results suggest the formation of template induced 1–1 diad repeats in the polymer main chain shedding unique light on the relative contributions of configurational and conformational imprinting.
Highlights
The molecular constituents of a living cell are predominantly water-soluble molecules carrying a net negative charge, e.g., cofactors, enzyme substrates, nucleic acids, the lipid double layer, and the glycocalix.[1−3] As a consequence, evolution has resulted in highly refined receptors capable of recognizing anions in water
Some of the most impressive examples are the proteins designed to selectively bind and/or transport either sulfate or phosphate ions. These two pyramidal anions are nearly isosteric with similar molecular volumes and central atom-oxygen bond lengths, whereas they differ with respect to Lewis basicity and hydrophilicity (Table 1)
Several synthetic receptors have been developed for this purpose to address various water processing or sensing applications,[6] but so far only few receptors have shown effective anion recognition in water and even more rare are those functioning in high ionic strength media.[1,51]
Summary
The molecular constituents of a living cell are predominantly water-soluble molecules carrying a net negative charge, e.g., cofactors, enzyme substrates, nucleic acids, the lipid double layer, and the glycocalix.[1−3] As a consequence, evolution has resulted in highly refined receptors capable of recognizing anions in water. Discrimination between phosphate and sulfate and other isosteric oxyanions (e.g., arsenate) occurs in the sulfate and phosphate binding proteins predominantly through multiple complementary hydrogen bonding (H-bond) interactions involving main chain amides (nests) in a water-poor microenvironment with minor involvement of charge complementary residues.[1,4,5] Notably, no charged residue is involved in the sulfate binding protein binding site which binds sulfate with an equilibrium binding constant Keq = 106 M−1 in water (5.0 ≤ pH ≤ 8.0),[5] an impressive feat given the strong hydration of this anion As a consequence, this overturns the Hofmeister series of the salting out tendency for anions which otherwise increases in the order; CH3COO− < HPO42− < SO42−
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