It is important to note that the absolute values of the Ka values for all BUs in this series (except for 12) are based on the Ka of BU 7 to chloride. Consequently, any absolute error in the determination of this Ka would result in a shift of the Ka of all BUs to either higher or lower absolute values, yet the selectivities of the BUs for chloride would remain unaltered. Cl -/HCO3 -transport studies in POPC liposomes The ability of the twelve BU derivatives to perform Cl -/HCO3 -antiport activity was investigated in large unilamellar vesicles (LUVs) using the lucigenin assay (Figure ). The chloride-sensitive probe lucigenin was encapsulated in the vesicles to monitor the influx of chloride anions into the LUVs mediated by BUs.