Structure-specific functionality of plant cell wall hydroxycinnamates

Within the plant cell wall, 4-hydroxy-3-methoxycinnamic acid (ferulic acid) has a clear role in polymer cross-linking. However, why this function appears largely restricted to the mono-methoxylated compound and not to other hydroxycinnamates appearing in the wall is less evident. Since radical coupling is the main mechanism by which hydroxycinnamate cross-linking occurs, the ease of parent radical formation and distribution of the unpaired electron were investigated. Ease of oxidation increased with increased substrate methoxylation, as did the amount of positive spin-density residing on the phenolic oxygen. The properties of the dimethoxylated hydroxycinnamate indicated that when esterified to the wall, coupling would result in C–O bond formation. This form of bonding is weaker and more flexible than the C–C bonding which would result from coupling of 4-hydroxy-3-methoxycinnamate and would not be desirable as a cross-link. Although the non-methoxylated compound could also couple via C–C bonds, ESR measurements of phenoxyl radical formation suggested that this compound would not easily participate in coupling reactions.