The peak at 589 nm for ATP-LW retained sensitivity to changes in concentration of ATP even after addition of ONOO -(16 μM) using excitation at 520 nm (Fig. ). The emission peak at 562 nm (Fig. ) or 569 nm (Fig. ) changed with the addition of ONOO -and was only slightly influenced by the addition of ATP (15 mM). Importantly, since the two emission peaks are close it becomes difficult to infer whether FRET is occurring between the naphthalimide and