Figure 4.

(Schematic diagram) Membrane-bound RH421. Two possible scenarios are shown that could both lead to a decrease in the fluorescence of RH421, as observed when either Na⁺ or K⁺ ions are occluded by the phosphorylated enzyme. The movement of either positive charge toward the amino end of the molecule (scenario a) or negative charge away from the amino end (scenario b) would both lead to redistribution of electrons within the ground-state RH421 chromophore toward the amino nitrogen and a shift in its fluorescence excitation spectrum to shorter wavelengths, i.e., higher energy.