The studies of the Cl -/NO3 -antiport activity described above had to be carried out at a higher concentration of 1 to obtain clear transport curves. This is in line with previous observations that the Cl -/NO3 -antiport process by fluorinated BUs is much slower than Cl -/HCO3 -antiport. This was attributed to the possibility of formation of a 2:1 complex of BU 1 with Cl -and HCO3 -anions, removing the need for full decomplexation of the BU during the transport process. Moreover, the high affinity of BU 1 towards NO3 - and selectivity for NO3 -over Cl -disfavour the decomplexation of NO3 -. The same trend was observed for HCO3 -/Cl -, which was significantly faster than HCO3 -/NO3 -antiport in experiments using the [Eu.L 1 ] + probe to directly monitor HCO3 -transport into LUVs. The results for the selectivities of transport processes by BU 1 obtained from these two assays show that the relative amounts or absolute concentrations of anions present in the medium do not play a significant role. For example, 10 mM HCO3 -was used in the [Eu.L 1 ] + assay (added to 225 mM NaCl), while 225 mM NaHCO3 was used in the lucigenin assay (to which 25 mM NaCl was added), resulting in similar rates of Cl -/HCO3 -antiport.