Protonation and second step of oxidation of Variamine Blue: Quantum-chemical treatment

It has been shown by MNDO, AMI, and PM3 calculations that, of the two possi ble conjugated acids of 4-amino-4'-methoxydiphenylamine (Varianime Blue), t he form protonated via the primary amino group is oxidized first. The catio n protonated via the primary amino group is more favorable energetically if hydration is taken into account. The detailed mechanism of the second step of oxidation of Variamine Blue was estimated based on the results of INDO, MNDO, AM1, and PM3 calculations (UHF formalism) of the charge distribution and spin density in the Variamine Blue radical cation.