Structural Characterization of the Lignin from the Nodes and Internodes of Arundo donax Reed Ana M. L. Seca, Jose ´ A. S. Cavaleiro, Fernando M. J. Domingues, Armando J. D. Silvestre, Dmitry Evtuguin, and Carlos Pascoal Neto* University of Aveiro, Department of Chemistry, 3810 Aveiro, Portugal Milled wood lignin (MWL) and dioxane lignin (DL) from different morphological regions (nodes and internodes) of Arundo donax reed were subjected to a comprehensive structural characterization by 13 C, 1 H NMR, FTIR, and UV spectroscopies and functional analysis. The permanganate and nitrobenzene oxidation methods were also applied to the in situ lignins. Both node and internode lignins are HGS-type lignins, with a significant amount of H units (including p-coumaric acid type structures). The S/G ratio (1.13-1.32), the weight-average molecular weight (20400-24500), the methoxyl group content (0.90-0.98), the phenolic hydroxyl group content (0.23-0.27), and the aliphatic hydroxyl group content (1.00-1.09) are not very different in the lignins from nodes and internodes. However, some structural differences between node and internode lignins were observed. The former has much more phenolic acids (p-coumaric and ferulic), 8.8% in node versus 1.2% in internode and less -O-4 (0.32 and 0.49 per aromatic unit in node and internode, respectively). In situ node lignin is more condensed than internode lignin. Keywords: Lignin; reed; Arundo donax; 13 C NMR; 1 H NMR; UV spectroscopy; FTIR spectroscopy; permanganate oxidation; nitrobenzene oxidation; phenolic acids; functional analysis; gel permeation chromatography INTRODUCTION In the past decades, fast growing plants have received particular attention as wood alternative sources of vegetable fibers. Arundo donax reed, a monocotyledon- ous plant with a segmented tubular structure like bamboo, growing naturally in Mediterranean countries with high biomass production rates (20-25 ton/ha/year) (Faix et al., 1989), is among those plants. Its potential applications include the use as a source of fibers for printing paper (De Coudens, 1966; Mela et al., 1994; Paavilainen and Hemming, 1995) and as a source of biomass for chemical feedstocks and for energy produc- tion (Faix et al., 1989). To improve the utilization of this plant, it is necessary to broaden the knowledge of structural features of its components. Previous chemical research on Arundo donax include chemical composi- tion, general features of macromolecular components (Pascoal Neto et al., 1997), and the structures of isolated hemicelluloses (Driss et al., 1973; Joseleau and Bar- noud, 1974, 1975, 1976). A few studies on the lignin of Arundo donax showed that it is composed of guaiacyl- and syringylpropane units with minor amounts of p-hydroxyphenylpropane units (Faix et al., 1989) and associated with phenolic acids (Tai et al., 1987). It was also shown that the lignin content in Arundo donax internodes greatly increases from the younger to the older parts of the plant (Joseleau and Barnoud, 1976; Joseleau et al., 1976). However, to our knowledge, no comprehensive work dealing with the complete struc- tural characterization of the lignin from the different morphological regions of Arundo donax has been re- ported until now. This paper reports the detailed chemical character- ization of milled wood lignin (MWL), dioxane lignin (DL), and in situ lignin from the nodes and internodes of Arundo donax by FTIR, UV, 1 H NMR, and 13 C NMR, spectroscopies, functional analysis, and chemical deg- radation methods. MATERIALS AND METHODS Preparation of Plant Material. The harvest of Arundo donax samples was performed as previously described (Pascoal Neto et al., 1997). The stems were separated from the foliage, air-dried, and cut in three fractions with the same length. The fraction corresponding to the middle part of the stem was used in this study. This part of the stem was manually further separated into nodes and internodes. The plant material was milled in a Retsch cross-beater mill SK1, sieved to 40 mesh, and air-dried. The plant powder was then submitted to successive extractions with petroleum ether, acetone, ethanol, and water (8 h each). Proteins were removed by treating extractive-free samples with 1% pepsin solution in 0.1 N HCl at 40 °C overnight, followed by hot water washing until neutrality. The lignin content was determined in extractive- and protein-free samples by the Klason method according to Tappi standard T 204 om-88 and found to be 17.2% for node and 19.8% for internode fractions (o.d. material). Isolation of Lignins. The milled wood lignin (MWL) was isolated from extractive-free plant powder. The MWL was isolated using a centrifugal ball mill (Retsch S1) with a sintered corundum I jar and balls and purified according to the Bjo ¨rkman method (Bjorkman, 1956) with minor modifica- tions (Obst and Kirk, 1988). The yield of the MWL obtained was 45.8% (of Klason lignin) for node and 25.2% for internode. The elemental analysis of MWL from node and internode gave 58.4% C, 6.4% H, 35.2% O and 57.0% C, 6.5% H, 36.5% O, respectively. The isolation of dioxane lignin (DL) from the internode region was based on the methodology described elsewhere (Pepper and Wood, 1962). Extractive- plus protein- * To whom correspondence should be addressed [fax +351 234 370084; e-mail cneto@dq.ua.pt]. 817 J. Agric. Food Chem. 2000, 48, 817-824 10.1021/jf9910988 CCC: $19.00 © 2000 American Chemical Society Published on Web 02/24/2000