Pectins of varying structures were tested for their ability to interact with cellulose in comparison to the well-known adsorption of xyloglucan. Our results reveal that sugar beet (Beta vulgaris) and potato (Solanum tuberosum) pectins, which are rich in neutral sugar side chains, can bind in vitro to cellulose. The extent of binding varies with respect to the nature and structure of the side chains. Additionally, branched arabinans (Br-Arabinans) or debranched arabinans (Deb-Arabinans; isolated from sugar beet) and galactans (isolated from potato) were shown bind to cellulose microfibrils. The adsorption of Br-Arabinan and galactan was lower than that of Deb-Arabinan. The maximum adsorption affinity of Deb-Arabinan to cellulose was comparable to that of xyloglucan. The study of sugar beet and potato alkali-treated cell walls supports the hypothesis of pectin-cellulose interaction. Natural composites enriched in arabinans or galactans and cellulose were recovered. The binding of pectins to cellulose microfibrils may be of considerable significance in the modeling of primary cell walls of plants as well as in the process of cell wall assembly.The well-known model of primary cell walls (PCWs) of dicotyledons emphasizes noncovalent interactions between cell wall polymers and suggests two independent, but interacting, networks where the cellulose-xyloglucan network is embedded in a matrix of pectic polysaccharides (Carpita and Gibeaut, 1993;Somerville et al., 2004).Cellulose, the primary structural element of the cell wall, is a homopolymer composed of (1/4)-linked b-D-Glcp residues. The linear chains of parallel alignment are tightly linked by hydrogen bonds to form microfibrils. Xyloglucan, the most abundant hemicellulosic polysaccharide in the PCWs of dicotyledons, is composed of a cellulose-like backbone consisting of (1/4)-linked b-D-Glcp residues, branched at O-6 by a-D-Xylp residues, which can be further substituted at O-2 by b-D-Galp residues (Fry, 1989). Some of the Galp residues may be substituted at O-6 by a-D-Fucp. Pectins are major components of dicotyledon PCWs and of the middle lamella. Their backbone is composed of smooth homogalacturonan (HG) and hairy rhamnogalacturonan (RG) I regions (O'Neill et al., 1990). HG, a linear chain composed of (1/4)-linked a-D-GalUAp units, can be methyl esterified at O-6 of carboxyl groups and acetyl esterified at O-2 and/or O-3 of secondary hydroxyl groups (Ralet et al., 2001). Some HGs might be substituted to form RG II or xylogalacturonan. RG II is a complex polysaccharide composed of GalUAp, Rhap, Galp, and some unusual sugars.Dimers of RG II were found to be cross-linked by two diester bonds through a boron atom (Fleischer et al., 1999). Xylogalacturonan contains b-D-Xylp residues attached to O-3 of the HG backbone (Le Goff et al., 2001). RG I contains a backbone of the repeating di- Renard et al., 1995). They are predominantly substituted at O-4 of Rhap residues by neutral sugar side chains (Schols and Voragen, 1994). The proportion of branched Rhap residues depen...

The structure of arabinan and galactan domains in association with cellulose microfibrils was investigated using enzymatic and alkali degradation procedures. Sugar beet and potato cell wall residues (called 'natural' composites), rich in pectic neutral sugar side chains and cellulose, as well as 'artificial' composites, created by in vitro adsorption of arabinan and galactan side chains onto primary cell wall cellulose, were studied. These composites were sequentially treated with enzymes specific for pectic side chains and hot alkali. The degradation approach used showed that most of the arabinan and galactan side chains are in strong interaction with cellulose and are not hydrolysed by pectic side chain-degrading enzymes. It seems unlikely that isolated arabinan and galactan chains are able to tether adjacent microfibrils. However, cellulose microfibrils may be tethered by different pectic side chains belonging to the same pectic macromolecule.

No abstract

We report a study of xyloglucan (XG)-cellulose interactions made possible by the preparation of various well-defined cellulosic and xyloglucosidic substrates. Bacterial microcrystalline cellulose (BMCC) as well as cellulose whiskers (CellWhisk) were used as cellulosic substrates. Xyloglucosidic substrates were obtained from Rubus cells and Tamarindus indica seeds. Different primary structure characteristics of XGs such as the backbone length and the nature of the side chains, as well as their repartition, were considered in order to examine the influence of the primary structure on their interaction capacity. Two complementary approaches were carried out: first, the determination of adsorption isotherms and its associated models, and second, an enthalpic study using isothermal titration calorimetry (ITC). This study highlighted that an increase of XG interaction capacity occurred with increasing XG molecular weight. Furthermore, we determined that a minimum of 12 glucosyl residues on the backbone is required to observe significant interactions. Moreover, both the presence of trisaccharidic side chains with fucosyl residues and an increase of unsubstituted glucosyl residues enhanced XG-cellulose interactions. The evolution of adsorption isotherms with temperature and ITC measurements showed that two different processes were occurring, one exothermic and one endothermic, respectively. Although the presence of an exothermic interaction mechanism has long been established, the presence of an endothermic interaction mechanism has never been reported.