Corroborating the abovementioned hypothesis, a similar halide over oxyanion selectivity has been observed with a Pd(II)-based anion transporter in which anion binding (via metal coordination) features a strong covalent character. Unfortunately, we were unable to determine the anion binding constants for the protonated PTSCs to evaluate the anion binding selectivity because of their instability. To quantify the Cl -vs NO3 -transport selectivity of 1, a pH gradient dissipation assay was conducted by applying a transmembrane pH gradient (pH 7.0 inside and pH 8.0) and measuring the rate of pH gradient dissipation under NaCl in /NaCl out and NaNO3 in /NaNO3 out conditions. These assays have been designed to evaluate H + /X -symport activities of anion transporters (Fig. ). The results demonstrate rapid H + /Cl -symport and no discernible H + /NO3 -symport from DMSO control baseline for transporter 1 used at the same concentration (Fig. ), confirming its extremely high Cl -> NO3 -selectivity at neutral to slightly basic pH. By using an osmotic response assay, we were able to determine H + /X -symport rates at pH 4.5 and found that transporter 1 remains Cl -> NO3 -selective under acidic conditions (Fig. ).