Chemistry and occurrence of hydroxycinnamate oligomers.

Hydroxycinnamates such as ferulic acid, sinapic acid and p-coumaric acid ester-linked to plant cell wall polymers may act as cross-links between polysaccharides to each other, but also to proteins and lignin. Although sinapates and p-coumarates also form cell wall cross-links by the formation of radically or photochemically formed dimers, ferulate derivatives are the quantitatively most important cross-links in the plant cell wall. While the first radically generated ferulate dimer was already identified almost 40 years ago, the spectrum of known ferulate dimers was considerably broadened within the last 15 years. Higher ferulate oligomers were generated in model systems, but also isolated from plant materials. Different model systems using either free hydroxycinnamic acids or their esters are reviewed, highlighting a discussion of the relevance of these models for the plant cell wall. The first ferulate trimer from plant material was discovered in 2003 and seven dehydrotrimers of ferulic acid were isolated from maize bran since. Some of these trimers were also identified in other plant materials such as wheat and rye grains, corn stover, sugar beet and asparagus. Formation mechanisms of ferulate trimers and implications for the plant cell wall are discussed. Ferulate tetramers are the highest oligomers isolated from plant materials so far. These compounds can theoretically cross-link up to four polysaccharide chains, assuming all cross-links are formed intermolecularly. Formation of intramolecular versus intermolecular polysaccharide cross-links is a key question to be answered in the future if we want to judge properly the importance of hydroxycinnamate cross-links in the plant cell wall. [ABSTRACT FROM AUTHOR]