Please do not adjust margins Chloride efflux in the presence of monensin was observed in all cases, and the results for receptors 4 and 9 are shown in Figure . In general, only minimal Cl -efflux was observed in the presence of valinomycin, indicating that the transporters do not function via an electrogenic Cl -uniport mechanism. The results in the presence of monensin demonstrate that the transporters function as H + /Cl -cotransporters. Compounds that facilitate a symport process have been shown to have potential for cancer treatment, by creating a pH gradient in tumour cells and inducing apoptosis. Compounds 9 and 10 were found to couple to valinomycin; however, efflux via this pathway was slower than when the compounds were coupled to monensin. Electrogenic Cl - transport requires the transporter molecule to diffuse back across the lipid bilayer as a neutral species after release of a chloride anion. This can be inhibited if a neutral receptor interacts strongly with phosphate headgroups in the bilayer, making them unable to diffuse across the membrane. Initial investigations on compound 1 in 2007 highlighted the potential for strong interactions between the receptor and phosphate. It is likely that this interaction is the reason that compounds 3-5 and 8 are incapable of facilitating Cl -uniport. Compounds 9 and 10 have the lowest Cl -binding constants of the anionophores tested in the cationophore coupled assay (Table ), and this suggests they likely also have the weakest affinity towards the headgroups. Therefore, compounds 9 and 10 can traverse the membrane as a neutral species and electrogenic Cl -transport is permitted; albeit at a slower rate than electroneutral H + /Cl - cotransport.