The two most active transporters of the series, 2c and 3c, had EC50 values of 0.0012 and 0.00043 mol%, respectively (Table ). Previously, the most active synthetic transporter reported using this assay was a 3,5-bis(trifluoromethyl)phenyl substituted o-phenylenediamine-based transporter (EC50 = 0.0015 mol%). The most active natural product tested using this assay was prodigiosin with an EC50 of 0.00064 mol%. Comparing the EC50 values obtained for transporters 2c and 3c shows that they are the two most active synthetic Cl -transporters reported using this assay. It is also of interest to compare 3c with A.P Davis' powerful anion transporters based on cholapod, decalin and anthracene scaffolds using specific initial rate analysis. The EC50 of compound 3c (delivered externally) can be converted to a specific initial rate of 600 s -1 assuming first-order kinetics. This value is comparable to the deliverability-adjusted specific initial rates of the best cholapod (670 s -1 ) and the best anthracenebisurea (1700 s -1 ). Such a comparison requires caution because different salt concentrations, lipid concentrations and lipid types were used. Based on the available data, however, it is likely that compound 3c has at least similar Cl -transport activities compared with the best cholapod and anthracene-based transporters, particularly noting that the Cl -/NO3 -exchange by 3c is rate-limited by NO3 -transport due to its small cavity being a poor fit for NO3 -as demonstrated by our anion selectivity studies (vide infra).