Compound 8 displayed similar voltage-dependent transport to the bis-urea compounds, however, the other thiourea compounds behaved differently under the conditions of the assays. Compounds 6 and 9 (Figure ) facilitated pH dissipation at a faster rate in the presence of a positive membrane potential, and when a negative membrane potential was applied the intravesicular solution became acidified. These findings suggest that compounds 6 and 9 can function as H + uniporters in the presence of valinomycin, because their neutral free receptor form is able to freely diffuse across the membrane which may be a consequence of weaker interactions with the phosphate headgroups.