Isomerization and tautomerism of the three water soluble vitamins including B3, B5 and B7 were studied applying density functional theory using B3LYP method in gas and aqueous phases.Activation energies (Ea), Gibbs free energies of activation (Delta G(#)), and imaginary frequencies of the transition state structures were calculated for all the isomerization and tautomerism reactions.Activation energies of the neutral? zwitterion (amine-enamine) tautomerism in vitamin B3 were 310-360 kJ/mol where these values for the keto-enol tautomerism were 100-130 kJ/mol.It was found that water molecule catalyzes the tautomerism and decreases the activation energies about 90-160 kJ/mol. (C) 2014 Elsevier B.V.All rights reserved.