Differences in stereo-preference in the oxidative degradation of diastereomers of the lignin model compound 1-(3,4-dimethoxyphenyl)-2-(2-methoxyphenoxy)-1,3-propanediol with enzymic and non-enzymic oxidants

Abstract

Mixtures of equal amounts of the erythro and the threo forms of the β-O-4 lignin model compound 1-(3,4-dimethoxyphenyl)-2-(2-methoxyphenoxy)-1,3-propanediol were oxidized with different ligninolytic enzymes and non-enzymic oxidants. The oxidants included cerium(IV) ammonium nitrate (CAN), Fenton's reagent, lead(IV) tetraacetate (LTA), laccase, laccase–mediator systems (based on the mediators ABTS, HBT, TEMPO, and VLA), and lignin peroxidase. The stereo-preference of the different oxidants was compared based on analyses of remaining substrate using HPLC and UV-diode array detector or 1H NMR spectroscopy. Fenton's reagent was the only oxidant tested that did not show preferential degradation of either the erythro or the threo form. CAN, LTA and lignin peroxidase preferentially oxidized the threo form. The stereo-preference of the laccase–mediator systems depended on the mediator. Oxidation mediated by HBT, TEMPO or VLA resulted in a preferential degradation of the threo form. Laccase/ABTS was the only system tested that showed preferential oxidation of the erythro form. The stereo-preference of the oxidants is discussed based on their redox potentials and their classification as outer-sphere and inner-sphere oxidants.