The principle of DPA/Tb3 for vesicle fusion assay is based on the fact that contact of the chelator dipicolinic acid (DPA) with terbium (III) forms an instant Tb3 -DPA complex that is ~10,000 times more fluorescent than free Tb3 . In the assay, separate vesicle populations are loaded with 2.5 mM TbCl3 in 50 mM sodium citrate, or 50 mM DPA in 20 mM NaCl.