Chlorination-Enhanced H⁺/Cl^- Co-Transport by Tyramine-Urea Conjugates

Engineering synthetic ionophores with precise control over the ion transport mechanism offers a powerful strategy for mimicking and modulating cellular processes. In this study, we designed a series of tyramine-urea conjugates (3-10) to achieve efficient H⁺/Cl^- co-transport by tuning their lipophilicity and anion-binding affinity. Single-crystal X-ray analysis of compound 4 revealed a 1:1 binding stoichiometry with Cl^- anion, wherein both phenolic -OH and urea -NH protons engage in H-bonding interaction with the anion. ¹H NMR titrations showed that thiourea derivatives exhibited stronger binding to Cl^- ions than urea analogues, likely due to their more acidic and stronger H-bond donating -NH protons. Introducing ortho-chloro substituents adjacent to the phenolic -OH group of the tyramine moiety effectively lowered its pK_a and enabled deprotonation-assisted co-transport of H⁺/Cl^- ions across lipid bilayers. While the non-chlorinated compounds 3-6 had EC₅₀ values in the 1.5-25.4 µM range, the chlorinated compounds 7-10 showed significantly enhanced ion transport activity with EC₅₀ in the 0.4-3.7 µM range. The most active transporter 9 followed a carrier-mediated pathway to symport H⁺/Cl^- ions across the bilayer membrane. Cell viability assays suggested a chloride-mediated cell death in cancerous cell lines, highlighting the potential of these tyramine-urea conjugates in anticancer therapeutics.