Erythro/threo ratio of β-O-4-5 structures as an important structural characteristic of lignin. Part 4: Variation in the erythro/threo ratio in softwood and hardwood lignins and its relation to syringyl/guaiacyl ratio

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Abstract
The proportion of erythro- and threo-forms of β-O-4-structures in lignin was elucidated by ozonation analysis of 21 wood species, and the relationship to the syringyl and guaiacyl composition was investigated. For all hardwood species, the erythro-form of β-O-4-structures predominated, although the extent varied widely, depending on wood species. In contrast, the proportion and amount of erythro- and threo-forms were very similar in all softwood species. The proportion of the erythro-form was greater in species with a higher methoxyl content in the lignin (correlation coefficient, R2=0.83). The S/V ratio (molar ratio of syringaldehyde and syringic acid to that of vanillin and vanillic acid) obtained by nitrobenzene oxidation was also strongly correlated with the proportion of the erythro-form (R2=0.99). Accordingly, the syringyl/guaiacyl ratio is closely related to the erythro/threo ratio. This stereochemical characteristic of β-O-4-structures is discussed in relation to the process of lignin formation.