Comprehensive Study on the Chemical Structure of Dioxane Lignin from Plantation Eucalyptus globulus Wood Dmitry V. Evtuguin,* ,† Carlos Pascoal Neto, † Artur M. S. Silva, † Pedro M. Domingues, † Francisco M. L. Amado, † Danielle Robert, ‡ and Oskar Faix § Department of Chemistry, University of Aveiro, 3810 Aveiro, Portugal, Centre de Recherche sur les Macromole ´cules Ve ´ge ´tales (CERMAV-CNRS), Domaine Universitaire, BP 53, 38041 Grenoble, France, and Bundesforschungsanstalt fu ¨ r Forst- und Holzwirtschaft, Institut fu ¨ r Holzchemie und Chemische Technologie des Holzes, D-21027 Hamburg, Germany Results of a comprehensive study on the chemical structure of lignin from plantation Eucalyptus globulus Labill are presented. Lignin has been isolated by a modified mild acidolysis method and thoroughly characterized by functional group analysis, by a series of degradation techniques (nitrobenzene oxidation, permanganate oxidation, thioacidolysis, and Py-GC-MS), and 1 H and 13 C NMR spectroscopy. Plantation Eucalyptus globulus lignin was found to be of the S/G type with an extremely high proportion of syringyl (S) units (82-86%) and a minor proportion of p-hydrophenyl propane (H) units (roughly 2-3 mol %). Unknown C-6 substituted and 4-O-5′ type syringyl substructures represent about 65% of lignin “condensed” structures. Eucalypt lignin showed high abundance of -O-4 (0.56/C 6 ) structures and units linked by R-O-4 bonds (0.23/C 6 ). The proportion of phenylcoumaran structures was relatively low (0.03/C 6 ). Different kinds of - substructures (pino-/ syringaresinol and isotaxiresinol types) in a total amount of 0.13/C 6 were detected. ESI-MS analysis revealed a wide molecular weight distribution of lignin with the center of gravity of mass distribution around 2500 u. Keywords: Lignin; Eucalyptus globulus; functional group analysis; nitrobenzene oxidation; permanganate oxidation; thioacidolysis; analytical pyrolysis; ESIMS; 1 H NMR spectroscopy; 13 C NMR spectroscopy INTRODUCTION Eucalyptus globulus Labill, a native dicotyledonous tree of southeast Australia, was introduced in the Iberian peninsula by the end of the 19th century, and later in Latin America, where soon it was recognized as a fast-growing tree with potential for pulp and paper production in those regions of the world. Nowadays, extensive short-rotation plantations (about 10-year rotations) of E. globulus may be found in Portugal and Spain, representing the major wood source for the production of market hardwood bleached kraft pulp in Europe. In Australia, where eucalypt trees can be found mainly in native forests, there is a recent growing interest in the use of plantation eucalypt (1). E. globulus represents, within this context, one of the most interest- ing species among the more than 600 species comprising the genus Eucalyptus. Despite this widespread interest and use of plantation E. globulus wood, literature on the detailed chemical and structural analysis of its components is quite scarce and dispersed, and it often refers to mature wood (30 years old and more) from native forests. The chemical composition of mature wood is known to differ signifi- cantly from that of young plantation eucalypt, and, hence, some care should be taken when making ex- trapolations from the chemical features of mature woods to those of young plantation woods. Previous chemical studies on plantation E. globulus wood included in-tree and between-tree variations in general chemical com- position (2, 3) and its relation with pulp quality (4) and kraft pulp yields (5). More detailed studies on chemical composition included extractives analysis (6-11) and neutral and acidic sugars composition (12). As far as lignin is concerned, two studies dealing with a brief characterization of isolated lignins from E. globulus wood have been previously reported (13, 14). However, the age and origin of the wood were not given. The tree- to-tree variation in the syringyl/guaiacyl ratio of E. globulus wood lignin was investigated by Rodrigues et al. (15) using analytical pyrolysis. The presence of polyphenols in eucalypt woods com- plicates the isolation, quantification (as Klason lignin), and structural analysis of lignins (16, 17). The milled wood lignin (MWL) from both mature (18) and planta- tion (19) eucalypt woods was obtained in poor yields and with a high proportion of attached hemicelluloses and tannins which hinder the quantitative analysis of the lignin structural elements. According to our results, the yield of MWL from eucalypt wood varies between 12 and 18%, whereas for the same milling conditions the yield of spruce MWL varies in the range of 30-35%. Good results on the purity of MWL from eucalypt woods were obtained by extraction of sawdust with alkali prior to the milling procedure (16). Dioxane lignins are fre- * To whom correspondence should be addressed (fax +351 234 370084; e-mail dmitry@dq.ua.pt). † University of Aveiro. ‡ CERMAV-CNRS. § Institut fu ¨ r Holzchemie. 4252 J. Agric. Food Chem. 2001, 49, 4252-4261 10.1021/jf010315d CCC: $20.00 © 2001 American Chemical Society Published on Web 08/08/2001