Abundance and Reactivity of Dibenzodioxocins in Softwood Lignin

• Published in: Journal of agricultural and food chemistry Vol. 50; no. 4; pp. 658 - 666
• Main Authors: Argyropoulos, Dimitris S, Jurasek, Lubo, Krištofová, Lívia, Xia, Zhicheng, Sun, Yujun, Paluš, Ernest
• Format: Journal Article
• Language: English
• Published: Washington, DC American Chemical Society 13-02-2002
• Subjects: Applied sciences; Carbon Isotopes; Exact sciences and technology; Heterocyclic Compounds - analysis; Heterocyclic Compounds - chemistry; Lignin - analysis; Magnetic Resonance Spectroscopy; Oxidation-Reduction; Paper, paperboard, non wovens; Phosphorus Radioisotopes; Polymer industry, paints, wood; Sulfides - chemistry; Thermodynamics; Wood; Wood. Paper. Non wovens; Phosphorus Isotopes; Lignin; Basic solution; Estimation; Carbon Isotopes; Softwood; Chemical degradation; Experimental protocol; Paper industry; Anions; Chemical concentration; Measurement method; Nuclear magnetic resonance; Chemical reactivity; Activation energy; Tracers
• ISSN: 0021-8561; 1520-5118
• DOI: 10.1021/jf010909g

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 31P 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 31P 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-β-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−β-aryl ether bond cleavage. Keywords: 5,5‘-Biphenyl; activation energy; derivatization followed by reductive cleavage (DFRC); dibenzodioxocin; HMBC; HMQC; model compound; kraft pulping; lignin; NMR; 31P; 13C; oxepine