Fig. | Intrachain electron transfer after double protonation. Shown is the change in total electron density induced by adding the second proton at the position marked by the red arrow in Double-1. The green arrow marks the position of the first proton. The decrease in total electron density occurs in the double bonds on the left side of the conjugated chain, cf. α-SUMO+1 in Fig. ). This is accompanied by long-range electron transfer to the position of the second proton as marked by the red arrow. Isovalue is set to 0.0015.