STUDIES OF LIGNIN BIOSYNTHESIS USING ISOTOPIC CARBON: VI. FORMATION OF THE SIDE CHAIN OF THE PHENYLPROPANE MONOMER

Abstract

A number of aromatic compounds having carbon side chains of varying length and substitution have been studied as lignin precursors. p-Hydroxybenzaldehyde was poorly utilized, indicating that the formation of C6,C3units from a condensation of C6,C1, and C2fragments is of very limited significance. The C6,C2compound, mandelic acid, was not converted to lignin. DL-Phenyllactic acid was incorporated into lignin with much lower dilution than either DL-erythro- or DL-threo-phenylglyceric acids, and with appreciably lower dilution than DL-phenylhydracrylic acid, supporting the concept that lignin monomers are formed via 2-hydroxylated phenylpropane intermediates. Wheat utilized the D-form of phenylalanine less readily than the L-form. Although three monocotyledons tested could utilize (+)- and (−)-phenyllactic acid to a comparable degree, two dicotyledons showed a marked preference for the (−)-form, suggesting that in dicotyledons the enzyme system specific for the (−)-form is predominant.