To rationalise the difference in ion selectivity between acyclic 1 and its cyclic variants 2-4, we computed the binding energies of Cl⁻ / OH⁻ ions to 1-4 at the SMD(CHCl3)-ωB97X-D3/def2-QZVP// SMD(CHCl3)-ωB97X-D3/def2-SVP level of theory . Chloroform was used to mimic the membrane dielectric in line with our previous work (see methods section and §7 SI for details). While such an implicit solvation model does not account for desolvation effects, it allows us to compare how binding preferences vary with transporter structure (Figure for compounds 1-2, Figures S50-S53 for 3-4).