Isolation and structural elucidation of norlignan polymers from the heartwood of Cryptomeria japonica

The norlignan (NorL) is a class of secondary metabolites, which occurs in the heartwood (hW) of certain softwood species. Although the NorL is often assumed to be secondarily altered (e.g., oxidized and/or polymerized in the hW over time), the formation and the chemical structure of oxidized/polymerized products remained unclear. In this study, we focused on the question whether an NorL oligomer/polymer exists in the hW of Cryptomeria japonica. First, NorL model polymers were prepared by horseradish peroxidase-catalyzed oxidation and their chemical structures were investigated by size exclusion chromatography-high-performance liquid chromatography (SEC-HPLC), nuclear magnetic resonance (NMR), and matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF-MS). Second, methanol (MeOH) extracted from the hW of C. japonica was fractionated, and one of the fractions contained NorL polymers as demonstrated by its chromatographic and spectrometric data in comparison with those of model polymers. Third, five kinds of agatharesinol (AGA) model dimers were synthesized and their chemical structures were determined. 13C-NMR signals corresponding to the model dimer structures were observed in the NMR spectrum of the natural polymer fraction. In summary, an NorL polymer was found in the hW of C. japonica, and its preliminary chemical structure was proposed for the first time.