Based on the hydration energies and the Hofmeister series, it was expected that the transport rates would be in the order I -> Br -> Cl -. Indeed, this trend was found when the transport experiments were done in NaNO3 solution. I -/NO3 -and Br -/NO3 -antiport processes were observed to be considerably faster (k of 0.013 and 0.023 s -1 respectively) than Cl -/NO3 -antiport (0.006 s -1 ) by BU 1 (Figure ). On the other hand, a rather surprising trend was found in the NaHCO3 solution (Figure ). The Cl -/HCO3 -antiport by 1 is the fastest (k of 0.126 s -1 ), followed by Br -/HCO3 -antiport (0.017 s -1 ) and very slow I -/HCO3 -antiport (0.004 s -1 ). Furthermore, the experiments with hydrophobic I -show that I -can permeate the LUV membrane to some extent even without any anion transporter (light purple curves in Figure ), meaning that part of the observed I -/HCO3 -antiport is due to spontaneous diffusion of I -, making this process slower than Cl -/NO3 -antiport. It is expected that due to such high affinities of BU 1 to anions, decomplexation does not occur at the membrane interface, but a direct exchange of the anions would be preferred (Figure ). In this case, a balanced selectivity of 1 for both anions involved in the antiport process would be beneficial. The anion binding selectivity of 1 in acetonitrile is following the order: I -> Br -> NO3 -> Cl -> HCO3 -(see Figure , Ka values provided in Table ). This would match the observation that Cl -/HCO3 -antiport, with both anions having similar Ka values (less than 30-fold difference) is faster than I -/HCO3 -antiport where a larger binding selectivity of BU 1 for I -is present (up to 2400-fold). Inversely, I -/NO3 -and Br -/NO3 -antiport are more efficient as both anions are very strongly bound in both cases. It was hinted previously for more structurally simple anionophores that a balanced selectivity for binding and transport of individual anions gives the fastest exchange rates in the antiport process. As mentioned above, F -can also be transported by 1 across lipid bilayers as investigated previously using the emissive probe [Eu.L 1 ] + directly monitor the concentration of F -inside LUVs (which cannot be done with lucigenin or SPBA). 85 F -uniport, F -/Cl -and F -/NO3 -antiport processes were found to be possible by 1 (Table ). Nevertheless, when considering halides, F -is the slowest to be transported, which might be linked to the low affinity that BU macrocycles have for F -compared to other halides in combination with the strong solvation of F -. Electrogenic uniport of Cl -, NO3 -or SO4 2-by 1 was investigated using the HPTS assay. LUVs of ~180 nm diameter were prepared from POPC and cholesterol in a 7:3 molar ratio with 1 preincorporated in the membranes at various concentrations. Instead of Na + , protonated N-methyl-ᴅ-glucosamine (NMDGH + ) was used as non-transportable counter cation for both the internal and external aqueous phases. LUVs with encapsulated HPTS were dispersed in NMDGH + solution with the counter anion of which the transport was to be studied (100 mM internal and external), containing HEPES (10 mM) at pH 6.8. The experiment was initiated by adding an NMDG solution (5 mM) to the exterior of the LUVs, creating a pH gradient across the vesicle membrane.