Cross-linking of arabinoxylans via 8-8-coupled diferulates as demonstrated by isolation and identification of diarabinosyl 8-8(cyclic)-dehydrodiferulate from maize bran
Introduction
Cereal grain arabinoxylans are important dietary fibre compounds and they also influence formation and properties of dough and bread (Andersson and Aman, 2001). As cell wall constituents of grasses in general, arabinoxylans are also an important source of energy for ruminants (Hatfield et al., 1999). The physicochemical properties of arabinoxylans are dependent on the arabinose/xylose ratio, the distribution of the sidechains, the degree of polymerisation, the extent of their cross-linking, and on their coupling to other polymeric cell wall components. Arabinoxylan cross-linking and cross-coupling to other cell wall components such as lignin or proteins is achieved via hydroxycinnamates, especially via ferulate and its derivatives (Bunzel et al., 2004a; Geissmann and Neukom, 1971; Markwalder and Neukom, 1976; Piber and Koehler, 2005; Ralph et al., 1994, Ralph et al., 2004). Ferulates acylate xylan arabinosyl units at their O-5-positions (Ishii, 1997).
Radical coupling of feruloylated arabinoxylans results in the formation of dehydrodiferulates or higher ferulate dehydrooligomers such as dehydrotriferulates and dehydrotetraferulates (Bunzel et al., 2003, Bunzel et al., 2005, Bunzel et al., 2006; Funk et al., 2005; Rouau et al., 2003). The ferulate moieties are coupled to produce new bonds that may occur at the 4-O-, 5- or 8-carbons, thus forming 5-5-, 8-8-, 8-5-, 8-O-4- and 4-O-5-linkages. Dehydrodiferulates are now routinely analysed in a whole range of plant materials following alkaline hydrolysis. Saponification, however, can modify the structures of the dehydrodiferulates involved. It is assumed that 5-5-, 8-O-4- and 4-O-5-dehydrodiferulic acids (DFA) are present as their ester analogs in the cell wall. This was also shown for 5-5- and 8-O-4-DFA by isolation and identification of 5-5- and 8-O-4-DFA-oligosaccharides following enzymatic or mild acidic hydrolysis (Allerdings et al., 2005; Ishii, 1991; Saulnier et al., 1999). From model reactions it is assumed that the 8-5(cyclic)-DFA is the natural form whereas the 8-5(open)- and 8-5(decarboxylated)-DFAs are saponification products (Ralph et al., 1994). The situation is less clear regarding the 8-8-coupled DFAs that are often the major dehydrodimers in cereal soluble fibers (Bunzel et al., 2001) and water-extractable rye arabinoxylans (Cyran et al., 2003). In addition to the formerly known dimers 8-8(cyclic)- and 8-8(open)-DFA, a third 8-8-coupled DFA, referred to as 8-8(tetrahydrofuran)-DFA, has been recently synthesised and identified in cereal grains (Schatz et al., 2006). Although we assume that all 8-8-coupled DFAs exist as such in the cell wall (Schatz et al., 2006), proving this assumption is of major interest. For example, studies to examine the ability of esterases to cleave various diferulate esters (Kroon, 2000; Wong, 2006) are only useful to the extent that model esters examined correspond to analogs present in the plant. The same is true concerning studies on the nutritional properties of ferulate dehydrodimers. 8-8-DFAs have been shown to be good antioxidants in copper-induced LDL oxidation test systems exceeding the activity of monomeric ferulate (Neudörffer et al., 2004). The 8-8(open)-dehydrodimer was shown to be more active than the cyclic counterpart also exhibiting different modes of action (Neudörffer et al., 2006). It is proposed that the structural integrity of wheat bran is increased by the presence of 8-8(cyclic)-DFA (Parker et al., 2005). Beyond the cereal grains, the 8-8(cyclic)-DFA is purported to play a key role in the thermal stability of Chinese water chestnut (Parker et al., 2003).
In this paper we describe the isolation and identification of a diarabinosyl 8-8(cyclic)-dehydrodiferulate from maize bran, proving that 8-8(cyclic)-dehydrodiferulates exist as such in the plant. In addition, we describe further 8-O-4-DFA-oligosaccharides implicating further xylosyl substitutions on the diferuloylated arabinose sidechains.
Section snippets
Plant materials
Maize bran (Zea mays L.) was kindly provided by Hammermühle Maismühle GmbH (Kirrweiler, Germany).
General
Heat-stable α-amylase Termamyl 120 L (EC 3.2.1.1, from Bacillus licheniformis, 120 KNU/g) was from Novo Nordisk (Bagsvaerd, Denmark). Amberlite XAD-2 was obtained from Serva (Heidelberg, Germany), Bio-Gel P-2 from Bio-Rad Laboratories (Hercules, CA, USA), and Sephadex LH-20 was from Pharmacia Biotech (Freiburg, Germany). Analytical (Nucleosil 100-5 C18 HD, 250×4 mm, 5 μm) and semipreparative
Isolation of 8-8(cyclic)- and 8-O-4-dehydrodiferuloyl saccharides
Maize bran, which was used in former studies to isolate feruloyl and dehydrodiferulolyl saccharides (Allerdings et al., 2005, Allerdings et al., 2006; Saulnier et al., 1995, Saulnier et al., 1999; Saulnier and Thibault, 1999), was chosen as the plant material in this study. Due to its high amounts of ferulates and dehydrodiferulates (Bunzel et al., 2001; Saulnier and Thibault, 1999) maize bran is an ideal starting material to investigate polysaccharide cross-linking in cereal grains. Acidic
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