Synthesis, Anion Recognition, and Transmembrane Anionophoric Activity of Tripodal Diaminocholoyl Conjugates

In this paper, we present the synthesis, anion recognition, and anionophoric activity of 1,3,5-tris(aminomethyl)-2,4,6-triethylbenzene-based tripodal 3α-hydroxy-7α,12α-diamino-5β-cholan-24-oate conjugate 1 and the corresponding tris(2-aminoethyl)amine-based analogue 2 and choloyl analogue 3. Their affinity toward anions was evaluated by means of competitive displacement assay using 5-carboxyfluorescein (5-FAM) as a fluorescent indicator. The results indicate compounds 1 and 2 exhibit strong recognition toward a wide range of biologically important anions, in particular, toward sulfate and phosphate anions. In MeOH-HEPES (4/1, pH 7), the binding constants of compounds 1 and 2 are 416- and 168-fold higher for sulfate than for chloride and 35- and 25-fold higher for phosphate than for chloride, respectively. The anion transport activity was measured by use of pH discharge assay and chloride-ion-selective electrode technique. The results indicate that compounds 1 and 2 function as effective anion-selective transporters in the order of ClO₄^- > I^- > NO₃^- > Br^- > Cl^- > SO₄^2- > H₂PO₄^- and exhibit anionophoric activity via a process of major anion exchange and minor anion/cation symport. In addition, some insights into the correlation of the anion binding affinity with the transport efficiency are also briefly discussed.