Azines provide an attractive balance in dynamic covalent chemistry, with rapid exchange in acidic conditions and good stability in the presence of water.

Synthetic anion transporters can be developed using anion receptors that are able to bind the anion and stabilize it in the lipophilic interior of a bilayer membrane, and they usually contain functional groups with acidic NHs, such as ureas, thioureas and squaramides. To assess the suitability of acylhydrazones as a new functional group for the preparation of anion transporters, we have studied a family of thioureas functionalized with these and related functional groups. ¹H NMR titrations and DFT calculations indicate that the thioureas bearing acylhydrazone groups behave as chloride receptors with two separate binding sites, of which the acylhydrazone binds weaker than the thiourea. Chloride transport studies show that the additional binding site has a detrimental effect on thiourea-based transporters, and this phenomenon is also observed for bis(thio)ureas with two separate binding sites. We propose that the presence of a second anion binding unit hinders the transport activity of the thiourea due to additional interactions with the phospholipids of the membrane. In agreement with this hypothesis, extensive molecular dynamics simulations suggest that the molecules will tend to be positioned in the water/lipid interface, driven by the interaction of the NHs of the thiourea and of the acylhydrazone groups with the POPC polar head groups and water molecules. Moreover, the interaction energies show that the poorest transporters have indeed the strongest interactions with the membrane phospholipids, inhibiting chloride transport. This detrimental effect of additional functional groups on transport activity should be considered when designing new ion transporters, unless these groups cooperatively promote anion recognition and transmembrane transport.

Stimuli-responsive transmembrane ion transport has become a prominent area of research due to its fundamental importance in cellular processes and potential therapeutic applications. Commonly used stimuli include pH, light, and reduction or oxidation agents. This paper presents the use of dynamic covalent chemistry to activate and modulate the transmembrane transport of chloride in liposomes. An active chloride transporter was obtained in situ within the lipid bilayer by dynamic azine metathesis. The transport activity was further tuned by changing the structure of the added azines, while the dynamic covalent chemistry could be activated by lowering the pH. This dynamic covalent chemistry opens a new approach towards controlling transmembrane transport.

Macrocyclization has proven to be a useful design strategy in the development of efficient anion receptors. In addition to ring size, the overall preorganization by structural rigidity is of key importance. To explore this in the context of developing an efficient pyrophosphate receptor, three macrocycles featuring a 26-membered interior ring size and similar H-bonding motifs have been synthesized and their anion binding ability investigated. Computational studies and NMR data showed different degrees of preorganization as a result of differences in flexibility. The interaction of the three macrocycles with chloride, dihydrogenphosphate, and dihydrogen pyrophosphate was investigated in solution by NMR and UV-Vis spectroscopy, and in the solid state by X-ray crystallography. The tetrahydrazone based macrocycle featuring intermediate flexibility exhibited the best affinity for all three anions investigated. Our results suggest that in addition to the proper preorganization of binding groups in a macrocycle a certain degree of flexibility is also required for an optimal affinity with the target guest.

The synthesis of phosphate receptors represents an important avenue of research given the ubiquity of phosphate in biological and environmental systems. While many molecular scaffolds suitable for smaller anions are available either commercially or via reported synthetic routes, scaffolds suitable for larger anions such as phosphate are less common. In this work, we present a clear and straightforward synthesis of the basic molecular scaffold 3,6-diaminophenanthrene and of a novel 3,6-bisureidophenanthrene anion receptor prepared from this scaffold. Of the seven synthetic steps using readily available starting materials and reagents, only a single chromatographic purification step was required. The different interactions of the 3,6-bisureidophenanthrene-based anion receptor with phosphate and chloride are demonstrated. We expect that this convenient synthesis of the 3,6-diaminophenanthrene building block will pave the way for applications in many different fields of research, from materials science to supramolecular chemistry.