1 H-NMR (500 MHz, DMSO-d6): δ 10.07 (s, 2H), 7.79 (dt, J = 7.7, 1.2 Hz, 2H), 7.74 (ddd, J = 9.9, 2.7, 1.6 Hz, 2H), 7.65 (dt, J = 7.4, 3.7 Hz, 2H), 7.57 (td, J = 8.0, 5.8 Hz, 2H), 7.44 (tdd, J = 8.5, 2.7, 1.0 Hz, 2H), 7.30 (dd, J = 6.1, 3.5 Hz, 2H). 1 H-NMR Binding Studies and Covariance of Fit Calculations Host solutions of tetralactams 1a-4c and acyclic compounds 5 and 6 (1 mM) were made up in DMSO-d6/H2O (0.5%). The same host solutions were then used to make the concentrated guest solution (1 M) containing the titrant guest anion (either Cl -or F -) as the TBA + salt. The host solution and the concentrated guest solution were then used to make a dilute guest solution (0.1 M). The titration experiments were initiated by first acquiring a 1 H-NMR spectrum of the host solution (0.5 mL) without any of the guest solutions added. Subsequent spectrums were acquired after each addition of the guest solutions. Initially, the dilute guest solution was added slowly until 10 equivs. of the guest anion was achieved. Further guest anion additions were done using the concentrated guest solution and only stopped after the interacting 1 H-NMR peaks of the host stopped shifting at ~400-600 Equivs. of the guest. The changing chemical shifts (ppm) of the resonances attributed to the protons of the host were first referenced to the DMSO-d6 proton peak at 2.50 ppm used as the internal standard and then tracked and inputted into the BindFit web applet from www.supramolecular.org to produce host:guest binding constants (Ka). The Cl -/NO3 -Exchange Assay and Hill Analysis The assay was performed using synthetic large unilamellar vesicles (200 nm) formed from POPC, which were rehydrated with an internal solution of NaCl (489 mM), and a phosphate buffer formed by Na2HPO4 (5 mM) and NaH2PO4•2H2O (5 mM). The vesicles then underwent nine freeze-thaw cycles. Once the vesicles had thawed to r.t., they were extruded through a 200 nm polycarbonate membrane twenty-five times and then dialysed in a phosphate buffer solution for 12 h to remove any residual unencapsulated Cl -. An external solution of NaNO3 (489 mM) was then made using the same phosphate buffer solution (5 mM), and both the internal and external solutions were adjusted to pH 7.2. The assay was prepared by first adding the vesicles (1 mM) into 5 mL of the external solution. Anion transport was initiated at t = 0 s by adding each tetralactam as a DMSO solution (10 μL), and the subsequent Cl -efflux was recorded using the ISE. The 100% Cl -efflux value required for data calibration was obtained by lysing the vesicles with Triton X-100 (50 μL, 10%, v/v) detergent at t = 300 s after which a further 120 s of data was collected before the transport experiment was stopped at t = 420 s. The recorded data was then calibrated to the obtained 100% Cl -efflux value recorded after t = 300 s. Each transport experiment at each receptor concentration was performed at least twice. If the transporters were soluble enough to be tested at a range of different concentrations, the collected datasets underwent Hill analysis to obtain the EC50 values and the Hill coefficients (n).