The tetralactam series were also investigated for their preference to transport a range of anions as NaX salts (100 mM, X = Cl -, Br -, NO3 -, I -, and ClO4 -) using the previously reported HPTS anion gradient transport selectivity assay. Vesicles containing Cl -were made the same way as the regular HPTS assay; however, the vesicles were suspended in different external solutions containing the anion of interest (Figure ). Transport was initiated when the 1a-4c were added as DMSO solutions where Cl -> X -transport causes the basification of the internal solution, resulting in a positive ΔpH, while X -> Cl -transport selectivity results in the acidification of the internal solution and a negative ΔpH (Figure , b). Equation -6 was then used to transform the fractional fluorescence intensity (If) data to the corresponding ΔpH value. Also, the greater the magnitude of the change of pH, the greater the preference of X -> Cl -transport is for the transporter being studied. For further details on methods, see the experimental procedures section, or for information on experimental data and spectra, see Figure -S163 in the ESI.