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Oxidative Coupling of Phenols and Anilines by Peroxidase: Structure-Activity Relationships¹

ABSTRACT

We investigated the relationship between molecular structure and reactivity for the peroxidase mediated coupling of substituted anilines and phenols. We undertook this study to gain a better understanding of the oxidative coupling processes operative in soil involving peroxidase enzymes. The relative reaction rates of the various substituted aniline and phenol compounds (electron donors) were determined by measuring electron donor disappearance using high pressure liquid chromatography. The effects of structure on reactivity were treated quantitatively by use of the Hammett equation. We found a good correlation between electron donor rate constants and their respective Hammett substituent ⁺ constants which indicated the formation of a positively charged transition state as the rate controlling step. The values obtained from these correlations were negative, demonstrating that reactivity was increased by the presence of electron-donating substituents on the aromatic ring which act to stabilize the positively charged transition state. These results suggested that the reaction rates depended on electron density at the reaction center (–NH₂ or -OH) in a predictable manner.

¹ Journal article no. 11090 of the Michigan Agric. Exp. Stn.

² Graduate Research Assistant and Assistant Professor, Dep. of Crop and Soil Sciences, Michigan State Univ., East Lansing MI 48824, respectively.

Received for publication September 6, 1983. Accepted for publication November 22, 1983.