A Mechanistic Basis for The Coupled Electron/Proton Transfer Reaction of Xanthine Oxidase: DFT Study. Computational Investigation (Gaussian 03) of the plausible coupled electron/proton transfer mechanism of Xanthine Oxidase in the presence of different substrates. The Becke-3-Lee Yang Parr (B3LYP) Density Functional Theory (DFT) calculation have been carried out using Gaussian 03 to probe the coupled electron/proton transfer mechanism.