Alkaline extractability of pectic arabinan and galactan and their mobility in sugar beet and potato cell walls Agata Zykwinska, Corinne Rondeau-Mouro, Catherine Garnier, Jean-Franc ¸ois Thibault, Marie-Christine Ralet * INRA, Unite ´ de Recherche Biopolyme `res, Interactions, Assemblages BP 71627, 44316 Nantes Cedex 03, France Received 30 November 2005; received in revised form 20 January 2006; accepted 8 February 2006 Available online 30 March 2006 Abstract The organisation of sugar beet and potato cell walls was studied using alkaline extractions following a response surface methodology, simultaneously with solid-state 13 C NMR spectroscopy. The influence of two extraction parameters: NaOH concentration (0.05, 0.275, 0.5 M) and temperature (40, 65, 90 °C) on the composition (neutral and acidic sugars) of the residues recovered was established. Treat- ments of increasing harshness progressively washed off non-cellulosic polysaccharides from the cell walls. Alkaline treatments applied to sugar beet cell wall material (SB-CWM) revealed the presence of diverse pectin populations. The existence of distinct pectin populations in potato cell wall material (P-CWM) was less outstanding. Solid-state 13 C NMR applied to SB-CWM and P-CWM and residues after treatment by 0.275 M NaOH at 65 °C revealed two fractions of pectic arabinan and galactan side chains. One fraction was highly mobile, whereas the other one displayed restricted mobility. Ó 2006 Elsevier Ltd. All rights reserved. Keywords: Pectin; Arabinan; Galactan; Xyloglucan; Experimental design; Solid-state 13 C NMR spectroscopy 1. Introduction Plant cell wall is a highly complex and dynamic structure that undergoes changes during cell division, expansion and differentiation (Carpita & Gibeaut, 1993). In dicotyledons cell walls, two different layers can be distinguished. The most external one, containing mostly pectic polysaccharides, con- stitutes the middle lamella. The inner one, characterised by a higher degree of organisation, is known as the primary cell wall. Primary walls of higher plant tissues are predominantly composed of polysaccharides (O’Neill & York, 2003). Pectins are complex carbohydrate polymers consisting of a backbone of (1 fi 4)-linked a-D-GalAp units. This linear homogalacturonan (HG) region is interrupted by a type I rhamnogalacturonan (RG I) consisting of the repeating disaccharide unit: (1 fi 2)-a-L-Rhap-(1 fi 4)-a- D-GalAp (Renard, Cre ´peau, & Thibault, 1995). A type II rhamnogalacturonan (RG II), a complex polysaccha- ride composed of GalA, Rha, Gal and some unusual sugars, constitutes also a part of the pectic molecule (Ishii & Matsunaga, 2001). RG II, although present as a quantitatively minor pectic subunit, is thought to play a key role in cell wall architecture (O’Neill, Ishii, Albersheim, & Darvill, 2004). Primary cell wall pectins are characterised by a high quantity of neutral sugar side chains mostly composed of Ara and Gal that are mainly branched at O-4 of Rha residues (Voragen, Pilnik, Thibault, Axelos, & Renard, 1995). Besides pectins, the other major polysaccharide of primary cell wall is cellulose. The cellulosic linear chains of parallel 0144-8617/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.carbpol.2006.02.012 Abbreviations: CP/MAS, cross polarisation magic angle spinning; C- WM, cell wall material; HG, homogalacturonan; P, potato; P-CWM, p- otato cell wall material; RG I, rhamnogalacturonan I; RG II, rhamnogalacturonan II; SB, sugar beet; SB-CWM, sugar beet cell wall material; SPE/MAS, single pulse excitation magic angle spinning. * Corresponding author. Tel.: +33 2 40 67 51 96; fax: +33 2 40 67 50 84. E-mail address: ralet@nantes.inra.fr (M.-C. Ralet). www.elsevier.com/locate/carbpol Carbohydrate Polymers 65 (2006) 510–520