The Erythro/Threo Ratio of β-O-4 Structures as an Important Structural Characteristic of Lignin. Part 2. Changes in Erythro/Threo (E/T) Ratio of β-O-4 Structures during Delignification Reactions

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Abstract
Summary: The ozonation method was applied to an analysis of the changes in erythro and threo (E/T) ratio of arylglycerol-β-aryl ether (β-O-4) structures in the residual lignins of delignified wood samples. When delignified by alkali cook, not only the total yield of erythronic and threonic acids per lignin decreased, but also the E/T ratio decreased with the progress of delignification. These results indicate that residual lignin of alkali-delignified wood contain only a small amount of arylglycerol-β-aryl ether structures and/or glycerol side chains, which are relatively rich in the threo form. A suggested reason for this, in addition to the stereo-preferential degradation of the erythroβ-O-4 structure, is the increasing contribution of topologically-inert lignin and/or chemically-inert lignin structures with a low original E/T ratio. In contrast, when the wood samples are delignified by NaClO2 treatment, no significant change is observed in the E/T ratio. This confirms that the NaClO2 delignification proceeds without the residual lignin retaining special configurational characteristics in its side chains.