The phenolic fraction of maize bran: evidence for lignin-heteroxylan association Catherine Lapierre a, *, Brigitte Pollet a , Marie-Christine Ralet b , Luc Saulnier b a Laboratoire de Chimie Biologique, INRA-INAPG, Institut National Agronomique, 78850, Thiverval-Grignon, France b Unite ´ de Recherche sur les Polysaccharides, leurs Organisations et Interactions, INRA, BP 71627-44316, Nantes Cedex 03, France Received 13 November 2000; received in revised form 22 February 2001 Abstract Maize bran heteroxylan samples were extracted in various conditions of severity. Their ferulate and diferulate content was investigated by GC–MS of methyl ester-TMSi derivatives. When extracted by 0.5 M NaOH in mild conditions, the heteroxylan sample contained a low level of ferulic acid (0.032% by wt.) and the main diferulate surviving alkaline extraction was found to be the 8–8 0 diferulate. On peroxidase treatment, this sample nevertheless produced a firm and brittle gel without any change in the diferulate profile. Typical lignin structures, mainly comprising syringyl units interconnected through b-O-4, b-1 and b-b interunit bonds,wereevidencedinthemaizebransample.Moreimportantly,theseligninstructureswerefoundtobetightlyassociatedwith the alkali-extracted heteroxylans. Thioacidolysis revealed the occurrence of 0.1–0.5% (by wt.) lignin structures in heteroxylan fractions extracted in mild or severe conditions, before and after purification of the polysaccharides. The gelling potential of the heteroxylan fractions was not only dependent on their ferulate level, but also influenced by associated lignin structures. These resultsarguefortheoccurrenceofcovalentlinkagesbetweenheteroxylanchainsandligninstructureswhichcouldparticipateinthe peroxidase-driven gelation of feruloylated polysaccharides. They demonstrate the role of low lignin levels in the organization of native or reconstructed polysaccharide networks. # 2001 Elsevier Science Ltd. All rights reserved. Keywords: Zea mays; Gramineae; Maize bran; Cell wall; Heteroxylans; Lignin; Polysaccharides; Ferulate; Dehydrodiferulate; Thioacidolysis 1. Introduction Maize bran is an important by-product of the maize industry. This cell wall fraction, mainly composed of matrixpolysaccharidesandcellulose,originatesfromthe pericarp tissues. The matrix polysaccharides, which represent more than 50% of the wall, are heteroxylans made primarily of xylose and arabinose units with minor amounts of galactose and glucuronic acid (Saul- nieretal., 1995a). In addition, cinnamic acids represent up to 4–5% by weight of the destarched maize bran (Saulnier et al., 1999; Saulnier and Thibault, 1999). These acids, mainly ferulic acid (3-methoxy-4-hydro- xycinnamic acid) together with a lower amount of p- coumaric acid (4-hydroxycinnamic acid), are esterified to the wall polymers (Kato and Nevins, 1985; Mueller- Harveyetal.,1986;Saulnieretal.,1995b).Manystudies with grass cell walls or appropriate ferulate model compounds have demonstrated that ferulate esters can participate in peroxidase-mediated oxidative coupling (Ralph et al., 1994; Wallace and Fry, 1995; Grabber et al., 1995). Such coupling results in the cross-linking of polysaccharide chains, thereby decreasing the wall extensibility(Kamisakaetal.,1990;Sanchezetal.,1996; Wakabayashi et al., 1997) and enzymatic degradability (Grabber et al., 1998). In addition, during lignification, the oxidative coupling between lignin units and ferulate esters anchors lignins to polysaccharides and further strengthens grass cell walls (Jacquet et al., 1995). In contrast to ferulic acid, p-coumaric acid is not prone to peroxidase-mediated coupling (Russel et al., 1996; Wal- lace and Fry, 1999), but participates in photodimeriza- tion reactions (Ford and Hartley, 1988). In addition to the biological importance of the ferulate-based cross- linking, the oxidative gelation of feruloylated pectins isolated from sugar beet (Thibault, 1986; Oosteveld et al., 1997) or of feruloylated heteroxylans isolated from 0031-9422/01/$ - see front matter # 2001 Elsevier Science Ltd. All rights reserved. PII: S0031-9422(01)00104-2 Phytochemistry 57 (2001) 765–772 www.elsevier.com/locate/phytochem * Corresponding author. Fax: +33-1-30-81-53-73. E-mail address: lapierre@grignon.inra.fr (C. Lapierre).