EPR detection of phytophenoxyl radicals stabilized by zinc ions: evidence for the redox coupling of plant phenolics with ascorbate in the H2O2-peroxidase system.

Chlorogenic acid (CGA; 3-o-caffeoylquinic acid), a phenylpropanoid metabolite of plants, was oxidized by H2O2 in the presence of horseradish peroxidase. The primary and secondary oxidized products both were free radicals which gave EPR multiline signals at g = 2.0044 and 2.0042 in the presence of zinc as a spin stabilizing agent. The EPR kinetics showed that ascorbate functioned as a cooperative reductant by regenerating CGA from its corresponding radicals. These results provide evidence to support the idea that the ascorbate-phenolic redox couple in conjunction with guaiacol peroxidase is an efficient H2O2 scavenging mechanism in higher plants.