In this context, we first evaluated the Cl -transport activity of 1, 2, 6, and 7 in Cl -/NO3 -exchange, using large unilamellar vesicles (LUVs, 200 nm, POPC:cholesterol 7:3, Fig. ) as a model membrane system. The kinetics of the exchange were followed by chloride-induced quenching of lucigenin emission (entrapped inside LUVs) in response to chloride pulses. Receptors 1, 2, 6, and 7 were incorporated into lipid bilayers either during (pre-incorporation method) or after (post-incorporation method) the formation of LUVs; in the latter case, the transporters were added to the suspension of vesicles as a solution in THF. The results show no significant transport activity for the receptors applied by the post-incorporation method. However, upon pre-incorporation, resorcin [4]arene 2 exhibited significant Cl -/NO3 - exchange activity, in striking contrast to 1, 6, and 7, which were still inactive (Fig. ). To quantify the transport activity of 2, we performed a series of experiments varying the 2-to-lipid molar ratio (Fig. ). This gave us the specific initial transport rate [I] = 88 s -1 , which is the highest among the CH-based transporters reported to date (much higher chloride transport activity was observed for fluorinated bambusurils in Cl -/HCO3 -antiport by Valkenier et. al., but the bambusurils were less active than 2 in Cl -/NO3 -antiport), and the EC50 value of 0.012 mol% (corresponding to 1:8333 transporter-to-lipid molar ratio), which confirmed the superior activity of 2 in Cl -/NO3 -exchange. Linear concentration dependence of initial transport rates (Fig. ) together with the Hill coefficient n = 1.0 ± 0.6 suggest that 2 acts as a monomer, and therefore likely operates as a carrier rather than as a channel, which is consistent with the fact that a single molecule of resorcinarene 2 (0.88 nm in hight) is too small to span the lipid bilayer (3.5-4 nm in thickness). The carrier mechanism was further corroborated by experiments in which the fluidity of the lipid membrane was varied, either by increasing cholesterol content, 64 or utilizing temperature-dependent fluidity of DPPC bilayers. Both experiments confirmed that transporter 2 operates as a carrier. For example, upon increasing the cholesterol content from 30 mol% to 45 mol%, a marked decrease in the transport rate was observed from (I) = 0.020 s -1 to (I) = 0.004 s -1 (see SI). Furthermore, in DPPC LUVs, which exhibit a phase transition at around 41°C, no transport activity of 2 was observed at 25°C and 35°C (i.e. below the phase transition temperature), but the activity was restored at 45°C, that is upon melting the membrane (Fig. ).