Abundance and Reactivity of Dibenzodioxocins in Softwood Lignin

Department of Chemistry, Pulp and Paper Research Centre, McGill University, 3420 University Street, Montreal, Quebec, Canada.
Journal of Agricultural and Food Chemistry (Impact Factor: 2.91). 03/2002; 50(4):658-66. DOI: 10.1021/jf010909g
Source: PubMed


To define the abundance and comprehend the reactivity of dibenzodioxocins in lignin, model compound studies, specific degradation experiments on milled wood lignin, and molecular modeling calculations have been performed. Quantitative (31)P NMR measurements of the increase of biphenolic hydroxyl groups formed after a series of alkaline degradations in the presence of hydrosulfide anions (kraft conditions) showed the presence of 3.7 dibenzodioxocin rings/100 C9 units in milled wood lignin. The DFRC degradation protocol (Derivatization Followed by Reductive Cleavage) was chosen as an independent means to estimate their abundance. Initial experiments with a dibenzodioxocin model compound, trans-6,7-dihydro-7-(4-hydroxy-3-methoxyphenyl)-4,9-dimethoxy-2,11-dipropyldibenzo[e,g][1,4]dioxocin-6-ylmethanol, showed that it is not cleaved under DFRC conditions, but rather it isomerizes into a cyclic oxepine structure. Steric effects precluded this isomerization from occurring when DFRC was applied to milled wood lignin. Instead, monoacetylated biphenolic moieties were released and quantified by (31)P NMR, at 4.3 dibenzodioxocin rings/100 C9 units. The dibenzodioxocin content in residual lignins isolated from kraft pulps delignified to various degrees showed that during pulp delignification, the initial rate of dibenzodioxocin removal was considerably greater than the cleavage rate of arylglycerol-beta-aryl ether bonds. The activation energy for the degradation of dibenzodioxocins under kraft conditions in milled wood lignin was 96 +/- 9 kJ/mol, similar to that of arylglycerol-beta-aryl ether bond cleavage.

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    • "The monolignols form structural elements when incorporated into the lignin polymer and are called p-hydroxyphenyl (H), guaiacyl (G), and syringyl (S) moieties, respectively. During the process of lignification, a complex three-dimensional polymer is produced through radical coupling of the moieties via b-O-4, a-O-4, b-5, b-1, 5–5, 4-O-5 and b-b linkages that do not form regular and ordered repeating units as found in cellulose (Argyropoulos et al. 2002; Froass et al. 1996; Kukkola et al. 2004). The amount and composition of lignin varies among different cell types, different taxa, and between individual cell wall layers. "
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    • "In earlier work we made extensive attempts to clarify the structural characteristics of residual lignin in kraft pulp (RKL, residual kraft lignin) to better understand the structural alterations that occur during pulping and bleaching processes (Jiang and Argyropoulos 1994; Granata and Argyropoulos 1995; Sun and Argyropoulos 1996; Asgari and Argyropoulos 1998; Argyropoulos and Liu 2000; Tohmura and Argyropoulos 2001; Argyropoulos et al. 2002a,b; Jä a ¨ skelä inen et al. 2003; Wu and Argyropoulos 2003). The coexistence of lignins and carbohydrates is an opportunity to study the nature of lignin carbohydrate bonds on the one hand, and a challenge when lignin analysis is the focus, on the other hand. "
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