Copper(II) complex, 1, with the histidine-derived ligand L (L = methyl 2-(2-hydroxybenzylamino)-3-(1H-imidazol-5-yl)propanoate) has been synthesized and characterized. Single-crystal structure determination reveals a diphenolato-bridged dicopper(II) core in 1. Addition of (NO)-N-center dot to an acetonitrile solution of 1 affords the corresponding mononuclear copper(II)-nitrosyl complex, 2. In the presence of H2O2, 2 results in formation of the corresponding copper(I)-peroxynitrite. Formation of peroxynitrite (-OONO) intermediate is evident from its characteristic phenol ring nitration reaction which resembles the tyrosine nitration in biological systems. Further, isolation. of nitrate (NO3-) as the decomposition product from 2 at room temperature also supports the involvement of -OONO intermediate.