Ferulic acid cross-linking

Some monocot, primary cell-wall polysaccharides may be cross-linked by esters of ferulic acid (4-hydroxy-3-methoxycinnamic acid). There is evidence for the existence of such cross-links in monocot tissues containing secondary walls, including Italia ryegrass stem276 and wheat endosperm.277 Ferulic acid is also present in barley cell-wall,278 and it has been reported that treatment with base releases ferulic acid from the cell walls of several Graminae,276 supporting the idea that ferulic acid is bound to the wall as an ester. However, ferulic acid has not, so far, been reported to be present specifically in primary cell-walls in either monocots or dicots. 

Suberins or polyestolides are related to cutins. These are complex polymers composed of co-hydroxy monobasic acids linked by ester bonds. They also contain a,P-dibasic acids esterified with diols, as well as ferulic and sinapic acid moieties. Suberins are enriched with molecules having 16 and 18 carbon atoms. They also have ethyl-enic and hydroxyl functionalities, and ester and ether cross-linking can occur. 

Piber, M., Koehler, P. 2005. Identification of dehydro-ferulic acid-tyrosine in rye and wheat Evidence for a covalent cross-link between arabinoxylans and proteins. J Agric Food Chem 53 5276-5284. 

Suberins. The cork cells in the outer bark contain polyestolides or su-berins. The suberin content in the outer layer of the cork oak bark (cork) is especially high and amounts to 20-40% in the periderm of birch bark. Polyestolides are complicated polymers composed of co-hydroxy monobasic acids which are linked together by ester bonds. In addition, they contain a,/3-dibasic acids esterified with bifunctional alcohols (diols) as well as ferulic and sinapic acid moieties. The chain lengths vary but suberins are enriched with molecules having 16 and 18 carbon atoms. There are also double bonds and hydroxyl groups through which ester and ether cross-links are possible. The outer layer of the epidermis contains so-called cutin, which is heavily branched and has a structure similar to suberin. 

In many plant species, the 05 of ot-L-Ara/ residues in plant cells walls is esterified with ferulic acid. One-electron oxidation results in cross-linking, both of feruloylated polysaccharides to each other, as with the terminal ot-L-Ara/ residues esterified with ferulic acid in the arabinan of rhamnogalacturonan 1 and to lignin (Figure 4.62). 

Films based on IPS, according to Gontard [9], the mechanical properties or barrier to water vapor unsatisfactory for practical applications, what became become worse under conditions of high humidity. Several studies have been made to improve the properties of the films based on IPS. It is reported that the addition of cross linking agents the film-forming solutions, or other physical methods, can improve the mechanical properties and/or the water vapor permeation (WVP) of films based on proteins [36]. Ou et al. [37] prepared films with 5.0 g of SPI, 3.0 g glycerol and different amounts (0, 50, 100, 150, 200 mg) of ferulic acid in two pH levels (8.0 and 9.0). At pH 9, the films showed better mechanical properties, as shown in Table 5.2. 

The other hpid polymer, suberin, is a heteropolymer, consisting of an aliphatic polyester associated with cross-linked polyaromatics and embedded waxes. Upon transesterification of suberin, the monomers released include C16-C28 m-hydroxy fatty acids and C16-C26 ct, -dicarboxyhc acids, the latter of which are diagnostic monomers, unsubstituted very-long-chain fatty acids (VLCFAs C>i8) and alcohols, glycerol and ferulate. Usually the major components of suberin are -hydroxy derivatives of palmitic and/or oleic acids, but in some cases oo-hydroxy C220 also is a dominant component [37]. Dicarboxylic FAs derived from further oxidation of the -hydroxy-FAs are also found in suberin. 

Cross-linking between polysaccharide-polysaccharide units can occur in different forms such as ester-linked hydroxycinnamate dimers (Ralph et al., 2004), ferulic acid and/orp-coumaric acid monomers as cyclic dimers (Grabber et al., 2004), and direct linkage between polysaccharides and xyloglucans (Harris and Stone, 2009). Some important interactions between lignin and polysaccharides in the cell walls are the following ... 

Arabinoxylans (Figure 3B) have a backbone composed of 1,4-linked P-D-Xylp residues—some of the Xyl/ residues may be 0-acetylated [7]. Ara/ residues are linked to position C-2 and/or C-3 of the backbone. In primary walls P-d-GIc/jA or 4-0-Me p-D-Glc A residues are also attached to the xylan backbone and these polysaccharides are called glucuronarabinoxylans. The Ara/ side chains of arabi-noxylan and glucuronarabinoxylan may also contain ester-linked phenolic acids such as ferulic acid [9] that are potential sites for cross-linking by oxidative coupling (Figure 3B and C). 

Figure 3. (A). A partial structure of a xyloglucan. The 1,4-liirked P-D-glucan backbone is substituted at C-6 with mono-, di-, and trisaccharides in a rather regular pattern. The xyloglucans present in the walls of the Solanaceae, Poaceae, and seed xyloglucans are not fucosylated. (B). A partial structure of a glucuronoarabinoxylan. The 1,4-linked (3-D-xylan backbone is substituted with arabinosyl and (4-0-Me) glucuronosyl residues, and 0-acetyl groups. Some of the arabinosyl residues are substituted with ester-linked phenolic acids such as ferulic acid. (C). Ferulic acid residues may be oxidatively coupled and thereby form inter- and intra-molecular cross links.

Figure 4. (A). A partial structure of homogalacturonan. In primary walls between 50 and 80% of the GalpA are esterified. (B). A partial structure of rhamnogalacturonan I (1) the backbone of RG-I is composed of the disaccharide repeating unit — 4)-a-D-GalpA-(l— 2)-a-L-Rhap-(l—The GalpA residues are often O-acetylated. Some of the rhamnosyl residues are substituted at C-4 with oligosaccharides composed predominantly of Ara/ and Galp residues (2 and 3). Some side chains contain (4-0-Me)GlcpA (4). In the Chenopodiaceae, the side chains are esterified with phenolic acids such as ferulic acid (5). Oxidative coupling of the phenolic acids may be a way in which pectins are cross-linked.

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