The absence of sulfate transport by most transporters allows the study of alternative transport mechanisms. For example, co-transport of propylammonium cations and chloride anions was demonstrated in Na2SO4. Furthermore, chloride uniport by anionophores can be studied in a K2SO4 solution in presence of the cationophore valinomycin. When either valinomycin or a chloride uniporter is present, no decay in fluorescence intensity should be observed, as a potential gradient would build up upon transport of cations or anions alone. The valinomycin is typically post-inserted into the membrane before the experiment at 0.1 mol% concentration with respect to the lipids (by addition of 5 µL of a 0.24 mM solution in methanol), ensuring fast transport of K + , so that chloride uniport by the anionophore would be rate-limiting.