Electrooxidative nitration of phenols at copper electrodes

Abstract

p-Cresol, as a small-molecule model for tyrosine residues in proteins, undergoes electrooxidative nitration in the presence of a nitrogen source, for example ammonia, in mildly alkaline aqueous solution at potentials in the range 0.8 to 1.4 V (vs sce). Anodized copper is the best electrode material of those studied and nitrogen sources in the increasing order of effectiveness are amides < amines ≅ proteins < ammonia, the latter giving a total of nitrocresols of ~ 30% from an initial p-cresol concentration of 0.5 mM. Azulene also nitrates in these conditions, but phenylethers (4-methyl methoxybenzene and 1,2-dimethoxy benzene) do not. The protein hen egg-white lysozyme (HEWL), in the absence of any other nitrogenous species, acts as a source of nitrating agent in the electrooxidative nitration of p-cresol thus substantiating our earlier finding of selective tyrosine nitration in proteins in the absence of any other nitrogen source. This electronitration reaction, unique in that there is no NO bond in any initial species, provides a novel and environmentally friendly route in mild conditions and is of particular benefit in the selective covalent modification of proteins.