Karolis Norvaisa

Synthetic anion receptors can transport anions across lipid bilayer membranes. Many anion transporters have been reported, including macrocyclic compounds. However, there are no systematic studies on how the size of macrocycles impacts their anion transport activity and selectivity. Therefore, we prepared eight amide-based macrocyclic compounds with ring sizes ranging from 18 to 26 atoms, as well as four bis-amides. We studied these compounds as anion receptors using chloride titrations and molecular modelling, and as anion transporters using liposomes with various fluorescent probes. Neither the size of the macrocycles nor the affinity for chloride were found to be determining factors for chloride transport activity; preorganisation appears to play a more important role. Fluorination was found to have a clear positive effect on anion transport rates, with the bis-amides performing surprisingly well compared to the macrocycles. The smallest non-fluorinated macrocycle exhibited selectivity for chloride over hydroxide and nitrate, whereas the pentafluorophenyl bis-amide demonstrated effective transport of bicarbonate and nitrate, likely due to anion-pi interactions in addition to hydrogen bonding. The insights from this study will shape the design of future anionophores.

The transmembrane transport of anions is a promising application of synthetic anion receptors. Numerous anionophores have been developed for chloride over the past decades. Despite the biological relevance of phosphate and phosphate esters, very few reports on their transport by synthetic systems exist. Here we report a systematic study on the transport of diphenyl phosphate, phenyl phosphate, and inorganic phosphate by five different anionophores. The transport of these phosphates into liposomes was monitored by fluorescence spectroscopy, 31P NMR spectroscopy, and an ion selective electrode. The results of these experiments showed that diphenyl phosphate is readily transported by most chloride ionophores. The transport of phenyl phosphate is more challenging, but can be enhanced by better shielding of the phosphate group. Inorganic phosphate is the most challenging to transport and was achieved using a macrocyclic anionophore with eight preorganised H-bond donors. These results pave the way for the development of anionophores for inorganic phosphate as well as phosphate esters.

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.

Synthetic anion receptors are increasingly explored for the transport of anions across lipid membranes because of their potential therapeutic applications. A considerable amount of research focuses on the transport of chloride whereas the transmembrane transport of inorganic phosphate has not been reported to date, despite the biological relevance of this anion. Here we present a calix[4]pyrrole with a bisurea strap that functions as a receptor and transporter for H2PO4–, relying on the formation of 8 hydrogen bonds and efficient encapsulation of the anion. Using a phosphate-sensitive lanthanide probe and 31P NMR spectroscopy, we demonstrate that this receptor can transport phosphate into vesicles by H2PO4−/Cl− antiport, H2PO4− uniport, and Cs+/H2PO4− sym-port mechanisms. This first example of inorganic phosphate transport by a neutral receptor opens perspectives for the future devel-opment of transporters for various biological phosphates.