The transporters were initially evaluated for their pH switchability by comparing their Cl -/NO3 - exchange activities under neutral and acidic conditions. POPC vesicles with internal NaCl (489 mM) buffered at pH 4.5 or 7.2 were suspended in a NaNO3 (489 mM) external solution buffered at pH 4.5 or 7.2. Anion transporters were added to the vesicles as DMSO solutions and Cl -efflux facilitated by an anion transporter was monitored using a chloride ion selective electrode (ISE). Similarly to previously reported PTSCs, transporters 1-9 were inactive Cl -/NO3 -exchangers at pH 7.2 but became "switched ON" to facilitate rapid anion exchange at pH 4.5 (Figs. 3, S20-S37). To quantify the pH switchability, concentration-dependent Hill plot analysis were conducted to give an effective transporter concentration that facilitates 50% of Cl -efflux at 270 s (EC50 values) at pH 4.5 and pH 7.2. As shown by Table , compound 1 was identified as the most active anion transporter, showing a low EC50 of 0.0175 mol% (with respect to lipids) at pH 4.5 and ~550-fold reduced activity at pH 7.2. For transporters 5 and 7-9, the Cl -/NO3 -exchange was almost completely suppressed at pH 7.2, with modest-to-high activities observed at pH 4.5. Compared with 1-3, the much weaker anion transport activities of 5, 8 and 9 under both pH conditions are attributed to the presence of electronwithdrawing substituents lowering the pKa of the iminium group or increasing the strength of the intramolecular hydrogen bond, thereby inhibiting imine protonation as required for anion transport. The low Hill coefficients observed for those transporters are due to the poor delivery of transporters to the membrane when high concentrations were used. In contrast to those transporters containing strong electron-withdrawing substituents, transporter 7 demonstrated a potent activity similar to 1-3 at pH 4.5 consistent with the modest electron-withdrawing ability of a single fluorine substituent at the paraposition. Transporter 6 precipitated upon addition to the vesicle suspensions as DMSO solutions which prohibited a detailed analysis of its pH switchability. With the pH-dependent Cl -/NO3 -exchange confirmed for the new PTSCs, we next investigated the Cl - uniport and H + /Cl -symport activities of transporters 1-9 at pH 4.5 and pH 7.2 using our previously developed cationophore-coupled ISE assays. These assays complement the Cl -/NO3 -exchange assay by providing additional insights into electrogenic/electroneutral transport phenomena and revealing the potentially slow NO3 -transport that could rate-limit the Cl -/NO3 -exchange process. POPC vesicles were loaded with KCl (300 mM) at pH 4.5 or 7.2, and suspended in an external solution containing inert K2SO4 (150 mM) at pH 4.5 or 7.2. Anion transporters were added along with a cationophore (valinomycin or monensin) to facilitate KCl efflux. In the presence of K + uniporter valinomycin, the anion transporter needs to facilitate Cl -uniport to give KCl efflux (Fig. bottom). By contrast, in the presence of K + /H + exchanger monensin, H + /Cl -symport facilitated by the anion transporter is required to generate net KCl efflux (Fig. ). Similar to prodigiosin, the previously reported PTSCs were shown to facilitate H + /Cl -symport (coupling to monensin) but no Cl -uniport (coupling to valinomycin) at pH 4.5. This was attributed to the inability of the cationic protonated PTSCH + (decomplexed transporter) to diffuse through the membrane without a bound anion. Back diffusion of the decomplexed transporter is a necessary step for Cl -uniport but not for anion exchange or H + /Cl -symport. Previously, the cationophore-coupled assays were not conducted at pH 7.2.