N-aryl-N-hydroxy urethanes as peroxidase substrates

N-Aryl-N-hydroxy urethanes (AHUs), which are promising N-OH mediators for oxidoreductase catalysis, are electrochemically active redox compounds with a redox potential of 0.35-0.40 V vs. SCE (0.59-0.64 V vs. NHE). Alkyl substituents in the carbamic acid residue have, as expected, only little influence on the redox potential. The highest potential was noticed for N-hydroxy-N-(4-cyanophenyl)-carbamic acid methyl ester. Recombinant Coprinus cinereus peroxidase (rCiP) catalyzes oxidation of the AHUs with apparent bimolecular constants k_ox of 2.5 · 10⁴-7.5 · 10⁴ M^-1 s^-1 at pH = 8.5 and 25°C. Structure-function connectivities of the substrates were analysed within the framework of the Marcus cross relationship and by using ab initio quantum chemical calculations. An excellent correlation of the redox potentials and the HOMO energies could be found. However, no correlation of log(k_ox) with redox potential and HOMO energy was indicated as predicted by theory. This was explained by specific docking of the substrates in the active center of rCiP.