The generally low solubility of 1a-4c in aqueous media caused difficulty in accurately assessing the transport activity under the required assay conditions and, in some cases, prevented the transport activity from being determined. Transporters 2a, 2c, 3a, 3c, and 4c showed a reasonable level of transport activity at concentrations low enough that precipitation did not occur, which allowed accurate data to be acquired. The data was fit to the Hill equation (Equation ) to obtain both the effective concentration of the transporter required to facilitate 50% of the total Cl -efflux (EC50) and the Hill coefficients (n, Table ).