In the absence of V, a small increase in emission intensity was also observed (Figure , blue curve). To test if 1 could act as H2PO4 -/M + symporter, the experiment was repeated, replacing K + with Na + or Cs + (Figure ). The higher response with Cs + (Figure , turquoise curve), compared to K + and Na + , can be attributed to H2PO4 -/Cs + symport. These results were not surprising given the reports on symport by calix [4]pyrroles, commonly with a selectivity for Cs + . 36, At pH 7, H2PO4 -and HPO4 2-are the main phosphate species present. Given the double negative charge of the dibasic HPO4 2- and its higher hydration energy (DGhyd = -1089 kJ mol -1 for HPO4 2- , 37 compared to -473 kJ mol -1 for H2PO4 ) we hypothesized that the phosphate transport observed by emission spectroscopy was due to H2PO4 -rather than HPO4 2-crossing the membrane. To verify this we set up a 31 P NMR assay, reminiscent of protocols used to monitor ion transport by Cl, 33 S, and C NMR spectroscopy, in which intra-and extra-vesicular phosphate can be distinguished upon the addition of Mn 2+ , a paramagnetic agent, to the exterior of the liposomes.