Selective cleavage of ether bonds in lignin

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Structural heterogeneity of lignin is one of the most important research targets for the full understanding of lignin structure and cell wall formation. In this study, different types of HASB reagents were applied to lignin for the selective and partial cleavage of ehter bonds, particularly beta-ether bonds in lignin. Treatment of periodate lignin with a limited amount of trimethylsilyl iodide gave a very small amount of low molecular weitht lignin fraction which was surprisingly rich in phenylcoumaran structure. In the case of pre-methylated periodate lignin, a very increased amount of low molecular weitht fraction was available which were basically formed by beta-ether linkages. These results strongly indicate the structural heterogeneity of lignin. It is olso interesting to note that the same treatment of a milled wood lignin (MWL) did not give any characteristic lignin fraction. All low molecular weight MWL fractions formed by this treatment tourned out to have quite similar characteristics. This means that MWL itself is a lignin fraction originated from a special morphological region of wood cell walls.Selective cleavage of ether bonds, in particular alpha-ether bonds, was also accomplished for unbleached kraft pulp. It is interesting to note that residual lignin in pulp became more easily extractable after the above treatment. Ether bonds between alpha-carbons of lignin side chains and carbohydrate must be one of the key factors of the resistant nature of lignin in pulp.