Heterogeneity in a water-extractable rye arabinoxylan with a low degree of disubstitution M. Nilsson a, * , R. Andersson a , R.E. Andersson b , K. Autio c , P. A ˚ man a a Department of Food Science, Swedish University of Agricultural Sciences, P.O. Box 7051 SE-750 07, Uppsala, Sweden b Department of Chemistry, Swedish University of Agricultural Sciences, P.O. Box 7015 SE-750 07, Uppsala, Sweden c Biotechnology and Food Research, VTT, FIN-02044 VTT, Espoo, Finland Received 5 January 1999; received in revised form 24 May 1999; accepted 1 June 1999 Abstract From rye grain samples, water extractable arabinoxylan with a low degree of disubstitution (AX I) was isolated by fractionation on a DEAE-cellulose column. AX I of all rye samples had a similar relative composition of un- and monosubstituted xylose residues and with less than 5% disubstitution. It showed a shear thinning behaviour and the viscosity was related to molecular weight. AX I isolated from different rye varieties had different viscosities at the same concentration. One rye sample had a weight average molecular weight of 125 000 (polydispersity 2) according to calculations from both light scattering and viscosity data. Signals from un- and monosubstituted xylose residues in 13 C NMR spectra were assigned using 13 C-HSQC-DEPT and COSY techniques. AX I probably consisted of blocks or different molecules with sparsely and more densely substituted regions.  2000 Elsevier Science Ltd. All rights reserved. Keywords: Water-extractable rye arabinoxylan; Low degree of disubstitution; Viscosity; NMR 1. Introduction Arabinoxylan is the predominant component of the diet- ary fibre complex in rye grain; hot water extractable arabi- noxylan constitutes about 30% of the total arabinoxylan content (Nilsson et al., 1997; Pettersson & A ˚ man, 1987), while room temperature water extractable arabinoxylan accounts for about 20% (Karlsson, 1988). Water extractable arabinoxylan from rye has previously been studied by Bengtsson and A ˚ man (1990), who found that approximately 50% of that arabinoxylan was a fraction they called AX I, an arabinoxylan with approximately 50% O-3 monosubstituted (1,4)-linked xylose residues in the backbone and with a small proportion of disubstituted xylose residues. An arabi- noxylan with a very high degree of disubstituted xylose residues (AX II) was isolated by enzyme degradation from a water extractable fraction and the authors hypothesized that water extractable arabinoxylan consists of a mixture of AX I and AX II type structures (Bengtsson, Anderson, Westerlund & A ˚ man, 1992). Later Vinkx, Delcour, Verbruggen and Gruppen (1995); Vinkx, Reaynart, Grobet and Delcour (1993) applied ammonium sulphate precipita- tion on water extracts from rye grain and isolated several fractions of arabinoxylan with Ara:Xyl ratios from 0.50 to 1.42; among them an intact arabinoxylan with a high proportion of disubstitution (60%) and also with monosub- stitution (14% at O-2 and 4% at O-3), hence confirming AX II as a polysaccharide present in water extracts of rye. Vinkx et al. (1993) on the basis of their fractionation and 1 H NMR studies, suggested that water extractable arabinoxylan consists of a range of structures. Viscosity of arabinoxylan in solution, like that of other polysaccharides, is dependent on molecular weight. Arabi- noxylan structure has also been shown to affect viscosity (Andrewartha, Phillips & Stone, 1979). Bengtsson et al. (1992) found a positive correlation between the proportion of disubstituted xylose residues and viscosity in water extracts of rye whole grain flour. Weight average molecular weight of rye water extractable arabinoxylan has been measured with gel permeation to 770 000 (Girhammar and Nair, 1992a). Little is known about molecular weight and viscosity of highly purified water soluble arabinoxylan from rye. In this study, we have isolated, from three different rye whole grains, a water extractable arabinoxylan with a low degree of disubstitution and studied their structure, molecular weight distribution and viscosity. The distribution of substi- tuents in a relatively large fragments obtained by incom- plete xylanase degradation of the arabinoxylan has also Carbohydrate Polymers 41 (2000) 397–405 0144-8617/00/$ - see front matter  2000 Elsevier Science Ltd. All rights reserved. PII: S0144-8617(99)00100-9 * Corresponding author. Tel.: + 46-1867-1597; fax: + 46-1867-2995. E-mail address: mathias.nilsson@lmv.slu.se (M. Nilsson) www.elsevier.com/locate/carbpol