N-arylhydroxamic acids as novel oxidoreductase substrates

N-Arylhydroxamic acids (AHAs) are promising novel N-OH mediators for oxidor eductase catalysis. They are electrochemically active compounds with a redo x potential of 0.31-0.41 V vs. SCE. Representative oxidoreductases, e.g. fu ngal peroxidase from Coprinus cinereus (rCiP), catalyze the oxidation of AH As with apparent bimolecular constants (k(ox)) of 7.1(.)10(3) to 1.5(.)10(7 ) m(-1)s(-1) at pH = 8.5 and 25 degreesC. The limiting step in substrate ox idation was the reduction of compound II (Cpd II), The oxidation constants of N-hydroxyacetanilide (1a) and N-hydxoxy-N-phenylbenzamide (2a), determin ed by a stopped-flow and steady-state method, were similar, The decrease in the reduction rate of Cpd II reduction rate decrease occurred at pK(a) = 8 .5 for 1a and 7.7 for 2a. The nitroxyl radical of 1a, an intermediate in th e oxidation, shows decreasing stability at alkaline PH. The structure-activ ity relationships (SARs) of these AHAs were analyzed within the framework o f Marcus cross-relationship, and by using ab initio quantum chemical calcul ations. A linear correlation of log(k(ox)) vs. redox potential was only ind icated for benzamides, as predicted by electron transfer theory. Acetamides showed the opposite tendency: the constant (k(ox)) increased if the potent ial of substrate increased, The correlation between log(k(ox)) and HOMO (Hi ghest Occupied Molecular Orbital) energy revealed that for benzamides the r eactivity decreased with decreasing HOMO energy, whereas for acetamides the reactivity increased if the HOMO energy decreased. The rather low reactivi ty of these AHAs and the unexpected dependence on the redox potential and t he HOMO energy could be explained by the electronic structure of the AHAs a nd the substrates docking in the active center of the enzyme.