Supramolecular Transmembrane Anion Transport: New Assays and Insights.

Transmembrane anion transport has been the focus of a number of supramolecular chemistry research groups for a number of years. Much of this research is driven by the biological relevance of anion transport and the search to find new treatments for diseases such as cystic fibrosis, which is caused by genetic problems leading to faulty cystic fibrosis transmembrane conductance regulator (CFTR) channels, which in turn lead to reduced chloride and bicarbonate transport through epithelial cell membranes. Considerable effort has been devoted to the development of new transporters, and our group along with others have been searching for combinations of organic scaffolds and anion binding groups that produce highly effective transporters that work at low concentration. These compounds may be used in the future as "channel replacement therapies", restoring the flux of anions through epithelial cell membranes and ameliorating the symptoms of cystic fibrosis. Less effort has been put into gaining a fundamental understanding of anion transport processes. Over the last 3 years, our group has developed a number of new transport assays that allow anion transport mechanisms to be determined. This Account covers the latest developments in this area, providing a concise review of the new techniques we can use to study anion transport processes individually without resorting to measurement of exchange processes and the new insights that these assays provide. The Account provides an overview of the effects of anion transporters on cells and an explanation of why many systems perturb pH gradients within cells in addition to transporting chloride. We discuss assays to determine whether anionophores facilitate chloride or HCl transport and how this latter assay can be modified to determine chloride versus proton selectivity in small-molecule anion receptors. We show how molecular design can be used to produce receptors that are capable of transporting chloride without perturbing pH gradients. We cover the role that anion transporters in the presence of fatty acids play in dissipating pH gradients across lipid bilayer membranes and the effect that this process has on chloride-selective transport. We also discuss how coupling of anion transport to cation transport by natural cationophores can be used to determine whether anion transport is electrogenic or electroneutral. In addition, we compare these new assays to the previously used chloride/nitrate exchange assay and show how this exchange assay can underestimate the chloride transport ability of certain receptors that are rate-limited by nitrate transport.