Isotope Effect Profiles in the Oxidative N-Demethylation of N,N-Dimethylanilines Catalysed by Lignin Peroxidase and a Chemical Model

Lignin peroxidase catalyses the oxidative N-demethylation of ring-substituted N,N-dimethylanilines by an electron-transfer mechanism whereby an anilinium radical cation is formed which is then deprotonated by the enzyme. Information on the nature of the basic centre which deprotonates the radical cation has been obtained by determining the KDIE profile (plot of k(H)/k(D), vs, the pK(a) of the aniline radical cations) for a number of ring-substituted N, N-bis(dideuteriomethyl) anilines. From the bell-shaped curve it has been estimated that the pK(a) of the proton-abstracting base is about 7. Interestingly, almost the same value has been obtained when the same type of study has been carried out using a water-soluble model compound: 5,10,15,20-tetraphenyl-21H,23H-porphine-p,p',p",p"'-tetrasulfonic acid iron(III) chloride. This is a strong indication that the radical cation is deprotonated by the same species in the enzymatic and in the chemical reactions. It is suggested that this species is the reduced iron-oxo complex.