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Enzyme-Catalyzed Complex-Formation of Amino Acid Esters and Phenolic Humus Constituents¹

ABSTRACT

The cross-coupling reaction between amino acid esters and phenolic humus constituents was investigated using a fungal laccase as a catalyst. A number of hybrid products were isolated and characterized by spectrometric analysis. When phenylalanine ethyl ester was combined with protocatechuic acid, the latter was oxidized to a quinone, which in turn coupled with the amino acid ester to form a quinonoid dimer. Two phenylalanine ethyl ester molecules coupled with a protocatechuic acid derivative through a single C-N bond (N-phenyl linkage), or through an imine linkage to form hybrid trimers. Similarly, the binding of phenylalanine ethyl ester to a syringic acid product took place at a carbonyl group via an imine linkage or at an aromatic C through a single C-N bond. The amino acid moiety and the phenolic compound in all trimers had a molecular ratio of 2:1. Our results provided clear evidence that enzymes can cause the linkage of amino acids to a lignin derivative by a covalent binding between amino N and an aromatic C. The chemical structures of some hybrid trimers also showed ring formation through a N atom to form a heterocyclic compound.

¹ Contribution from the Laboratory of Soil Microbiology, Dep. of Agronomy and Dep. of Chemistry, Pennsylvania State Univ., University Park, PA 16802. Journal Series no. 6893 of the PA. Agric. Exp. Stn. This work was partially supported by the Office of Research and Development, Environmental Protection Agency (Grant no. R-808165).

² Research Assistants, Lecturer and Professor of Soil Microbiology, respectively.

Received for publication April 16, 1984. Accepted for publication October 23, 1984.