Chemical Structure and Biosynthesis of the Aromatic Domain of Suberin in Bark

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Suberin is one of the main components in the outer bark of woody plants. The insoluble biopolymer is composed of an aliphatic domain and an aromatic domain. Although the structure and composition of the aliphatic domain have been well investigated, those of the aromatic domain are unclear. Furthermore, there have been no study on biosynthesis of the suberin aromatic domain. In this study, first p-hydroxycinnamic acid alkyl esters, as predicted precursors of the aromatic domain of suberin, in the bark of Quercus variabilis and Ginkgo biloba were analyzed. Caffeic acid alkyl esters and ferulic acid alkyl esters were specifically present in the outer bark of Q. variabilis, while ferulic acid alkyl esters were specifically present in the outer bark of G. biloba. Second, dehydrogenative polymerization of butyl ferulate and butyl caffeate with crude enzymes from both of the outer bark in the presence of hydrogen peroxide were carried out as a model system. The polymerization of butyl ferulat … More e gave dimers of β-O-4' enol ether, β-5', dihydronaphthalene, naphthalene, and β-β' symmetry types, and that of butyl caffeate afforded dimers of dihydronaphthalene, naphthalene, and β-6' types. Third, methanolysis of extractive-free bark powder from both tree species was performed. Methyl ferulate dimers of β-O-4' enol ether and β-5' types, feruloyloxy fatty acid methyl esters, methyl ferulate, vanillin, and coniferaldehyde were isolated and identified from Q. variabilis. The β-O-4' dimer and feruloyloxy fatty acid ester were obtained from G. biloba. Fourth, individual administrations of L-[U-^<14>C]phenylalanine and p-hydroxy[8-^<14>C] cinnamic acid to excised branches of Q. variabilis, followed by methanolysis of the bark administered were carried out. The radioactivity was incorporated into fractions that contain β-O-4' dimer and coniferaldehyde. Finally, to isolate polymeric suberin aromatic domain/lignin from both of the outer bark and to analyze the structure by FT-IR and solid-state ^<13>C NMR, extractive-free fine powder of the outer bark was prepared and treated with cellulases, hemicellulases, and pectinases, followed by methanolysis. It was suggested that the polysaccharides in the outer bark have chemical bonds with suberin aliphatic domain rather than with the aromatic domain. These results indicate that ferulic acid derivatives are dehydrogenatively polymerized by peroxidase to give the suberin aromatic domain in the outer bark of Q. variabilis and G. biloba. Less