4 - Phenolic acid dimers in barley straw cell walls

Abstract

We report the tentative characterization of three groups of alkali soluble 3-OH-4-CH3 substituted phenolic acid dimers in the cell walls of barley straw. The two predominant groups are derived from ferulic acid by photochemical (truxinic/truxillic acids; Fig. 1A) and oxidative (aryl-aryl linked; Fig. 1B) dimerization. Within the latter group six compounds were identified by GC/MS as having mass spectral fragmentation patterns consistent with ring-linked dehydrodiferulic acids. Assuming the absence of isomerism about the aryl-aryl bond this means that linkages other than 5-5′ are present. Due to steric constraints we propose that the other linkage is 6-6′. The third group of dimers appears to be derived from 3(4-hydroxy-3-methoxyphenyl) propanoic acid (hydroferulic acid; Fig 1C). Four dimers were present at the limits of detection by GC/MS. This again suggests that linkages other than 5-5′ are present. No monomelic hydroferulic acid was found. Analysis of the cell walls at various stages of growth showed that, in general, the number of dimer types accumulated with age, and at a given age, were present more in the basal than the apical tissue. Neither truxillic nor truxinic acid derivatives were detected in etiolated tissue, supporting their proposed photochemical origin. Digestion of mature barley cell walls with a purified xylanase followed by alkaline extraction of the soluble digest released the same range of dimers identified after extraction with 1M NaOH alone. Fractionation of the digest using Sephadex LH-20 and subsequent 1M NaOH extraction and GC/MS analysis showed that the dimers were absent from the fraction eluted with water, but, elution with 50% 1,4 dioxane provided the full complement of dimers. This suggests that the dimers, in the digest are attached to relatively small oligosaccharide fragments.