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.
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Liu et al. (1985) studied this question.