Transmembrane Transport of Bicarbonate by Anion Receptors

The development of synthetic anion transporters is motivated by their potential application as treatment for diseases that originate from deficient anion transport by natural proteins. Transport of bicarbonate is important for crucial biological functions such as respiration and digestion. Despite this biological relevance, bicarbonate transport has not been as widely studied as chloride transport. Herein we present an overview of the synthetic receptors that have been studied as bicarbonate transporters, together with the different assays used to perform transport studies in large unilamellar vesicles. We highlight the most active transporters and comment on the nature of the functional groups present in active and inactive compounds. We also address recent mechanistic studies that have revealed different processes that can lead to net transport of bicarbonate, as well as studies reported in cells and tissues, and comment on the key challenges for the further development of bicarbonate transporters.

A Pyridyl-Linked Benzimidazolyl Tautomer Facilitates Prodigious H+/Cl- Symport through a Cooperative Protonation and Chloride Ion Recognition

We report two pyridyl-linked benzimidazolyl hydrazones as HCl cotransporters that are 5 and 2 times superior to prodigiosin, a natural product whose transport efficiency has never been routed by synthetic molecules. These hydrazones provide a suitable HCl binding site through a cooperative protonation and chloride ion recognition. HCl transport by the most active compound induces lysosome deacidification. Viability assays confirmed that the compounds induce cytotoxicity toward human breast cancer MCF-7 cells but are relatively nontoxic toward noncancerous HEK293T cells.

Transmembrane Transport of Bicarbonate Unravelled*

Anion receptors can be used to transport ions across lipid bilayers, which has potential for therapeutic applications. Synthetic bicarbonate transporters are of particular interest, as defects in transmembrane transport of bicarbonate are associated with various diseases. However, no convenient method exists to directly observe bicarbonate transport and study the mechanisms involved. Here, an assay is presented that allows the kinetics of bicarbonate transport into liposomes to be monitored directly and with great sensitivity. The assay utilises an encapsulated europium(III) complex, which exhibits a large increase in emission intensity upon binding bicarbonate. Mechanisms involving CO2 diffusion and the dissipation of a pH gradient are shown to be able to lead to an increase in bicarbonate concentration within liposomes, without transport of the anion occurring at all. By distinguishing these alternative mechanisms from actual bicarbonate transport, this assay will inform the future development of bicarbonate transporters.

Fixation of atmospheric CO2 as novel carbonate-(water)2-carbonate cluster and entrapment of double sulfate within a linear tetrameric barrel of a neutral bis-urea scaffold

A meta-phenylenediamine-based disubstituted bis-urea receptor L1 with electron-withdrawing 3-chloro and electron-donating 4-methylphenyl terminals has been established as a potential system to fix and efficiently capture atmospheric CO₂ as air-stable entrapment of an unprecedented {CO₃^2--(H₂O)₂-CO₃^2-} cluster (complex 1a) within its tetrameric long straight pillar-like assembly entirely sealed by n-TBA cations via formation of a barrel-type architecture. L1 and its isomeric 4-bromo-3-methyl disubstituted bis-urea receptor L2 have been found to entrap similar kinds of water-free naked sulfate-sulfate double anion (complexes 1b and 2a) by cooperative binding of urea moieties inside the two pairs of the inversion-symmetric linear tetrameric barrel of L1 and L2, respectively. On the other hand, in the presence of excess halides, L1 self-assembles to form hexa-coordinated fluoride complex 1c and tetra-coordinated bromide complex 1d, while L2 self-assembles to form penta-coordinated fluoride complex 2b in the solid state via semicircular receptor architectures and non-cooperative H-bonding interactions of urea moieties.

Dissecting the chloride–nitrate anion transport assay

The chloride/nitrate selectivity of anion transporters in both binding and membrane transport is examined revealing the limitations of chloride–nitrate anion exchange assay.