Relationships between blacking phenomenon and norlignans of sugi (Cryptomeria japonica) heartwood III: Coloration of norlignans with alkaline treatment

Abstract

We investigated norlignan coloration with alkaline treatment to clarify the relationship between the blackening of sugi (Cryptomeria japonica D. Don) heartwood and norlignans in the heartwood. Of the four main norlignans (agatharesinol, sequirin-C, sugiresinol, hydroxysugiresinol) of sugi heartwood, only sequirin-C was clearly colored with alkaline treatment (0.4% potassium hydrogencarbonate solution, pH 8.6). Sequirin-C changed color to deep purple with alkaline treatment. The absorbance spectrum of colored sequirin-C had two peaks (450 and 525nm) and one shoulder (626nm). Coloration began at pH 6.2.). The spectrum of blackened sugi heartwood was similar to that of sequirin-C treated with alkali. A catechol nucleus and a double bond conjugated with a benzene nucleus play important roles in norlignan coloration with alkaline treatment. The number of hydroxyl groups is related to solubility in alkaline solution, and, therefore, whether a norlignan changes color also depends on the number of hydroxyl groups. Based on these results, we concluded that sugi heartwood turns black because sequirin-C, which is readily soluble in alkaline solution and can form a large intramolecular conjugation system when alkalized, is converted to products with a deep purple color as the heartwood is basified. © The Japan Wood Research Society 2006.