Chemical, Enzymatic, and Electrochemical Oxidation of Biophenols

Abstract

The redox properties of phenols can be studied using chemical, enzymatic, and electrochemical techniques. Whereas chemical and enzymatic methods have fixed oxidation potentials, electrochemical methods allow for control of the oxidation potential. Here, the oxidation product outcomes from KMnO4 oxidation, peroxidase/HRP oxidation, and bulk electrolysis of 4-hydroxyphenylacetic acid (HPA) are presented and compared with HPLC. ESR data is also shown. Similar data is presented for other biologically relevant phenols.

Introduction

HPA is a monocarboxilic acid and a member of the phenol family. In industry, HPA serves as a common precursor to the synthesis of drugs and pesticides due to its simplicity in structure (figure 1) (1,2). HPA is an important metabolite for anaerobic and aerobic bacteria alike....(3,4) As a phenol, HPA is able to undergo a one electron oxidation with an appropriate oxidizing agent to generate the radical shown in figure 2. This oxidative radical will couple to other molecules to form dimers, trimmers, and heavier polymers. HPA will readily undergo radical polymerization when met with the appropriate conditions. Common oxidation techniques employed to generate the radical include doing so chemically, electrochemically, and enzymatically.

Electrochemical, enzymatic, and chemical oxidation data is shown for HPA in this paper and the oxidation product outcomes are compared. Initial data for the more complex tyrosine and N-acetyl tyrosine is also presented.

Materials and Methods

Chemical Oxidation

Chemical Oxidation of 2.00 mM HPA (pH 5) in H2O was carried out with KMnO4 at various concentrations ranging from 0.33 mM to 4.00 mM. Reactions were carried out in 5 mL reaction vials.

Beaker Enzymatic Oxidation

Enzymatic Oxidation of 2.00 mM HPA (pH 5) in H2O was carried out with horseradish peroxidase (HRP) and various concentrations of H2O2 ranging from 0.25 mM to 2.00 mM. HRP concentration was kept constant at 500 nM for each reaction. Reactions were carried out in 5 mL reaction vials. Enzymatic oxidation of N-acetyl tyrosine was carried out under the same conditions described for HPA. Enzymatic oxidation of tyrosine was completed with the same experimental parameters as decribed for HPA and NAY; however, tyrosine was studied at 6.00 mM and solubilized with 1 mL 0.1 M NaOH prior to addition to pH 7.4 H20 solution due to solubility issues.

Discussion

One of the main takeaways from the oxidation data of HPA is the oxidation product outcomes are not only dependent on generation of the radical; this is also dependent on the oxidation technique employed. If product outcome was only depent on generation of the radical, then the oxidation product distribution would look the same gor each method, but this is not the case.

Page History

This page was created by Sara L. Simonson in the fall of 2021