Chemistry and occurrence of hydroxycinnamate oligomers

Abstract

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 crosslinks 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.

Phenolic Compounds in Maize Grains and Its Nixtamalized Products

“…However, the abundance of each one varied according to the maize type [18]. Most of the ferulic acid in the pericarp of the maize grain is linked by ester bonds to cell wall polysaccharides [35]. Nonetheless, they are also present in the form of dehydrodimers originating from the oxidative coupling of ferulate esters by means of the enzyme peroxidase.…”

Structural characterization of allomelanin from black oat

“…Our results are novel not only as concerns melanin, but for other reasons too. Among the hydroxycinnamates, only ferulate oligomers (dimers, trimers and tetramers) have already been isolated from plants (maize, wheat and rye) (Bunzel, 2010). They act as cross-links between the cell wall polymers (polysaccharides, proteins and lignin), contributing to the strengthening of the cell wall and the defense mechanisms of the plants against pathogens.…”

“…This structural difference was attributed to the surface organization and orientation of the monomer. In plants, phenolic acids occur partially in free form, but they are mostly ether-linked to lignin through their OH groups and ester-linked to structural carbohydrates and proteins through their carboxyl groups (Bunzel, 2010). It may be surmised that the orientation of p-coumaric acid by the cell wall polymers contributes to the formation of the C-C linkages between the aromatic rings.…”

Thermostable feruloyl esterase for the bioproduction of ferulic acid from triticale bran

“…Besides, FA can be converted enzymatically to various value-added products, e.g., vinyl guaiacol and vanillin that are known flavoring agents in foods, beverages, or perfumes (Rosazza et al 1995;Mathew and Abraham 2006). Present in both monocots and dicots of the plant kingdom, FA is generally not found in free form but instead esterified to arabinose in various polysaccharides, e.g., arabinoxylans and pectins (Carnachan and Harris 2000;Bunzel 2009). Ferulic acid esterases (FAEs, also known as feruloyl esterases, cinnamic acid, or cinnamoyl esterases; EC 3.1.1.73) represent a diverse group of hydrolases that can release FA from its bound constituents in the plant cell wall.…”