Heterogeneous molar mass distribution of chromophores in milled wood lignins isolated from four lignocellulosic species

Abstract Lignin, a major component of lignocellulosic cell walls, is an aromatic polymer known to exhibit a high molar absorption coefficient and fluorescence properties in various solvents and polymeric environments. Understanding these fluorescence characteristics is crucial not only for developing lignin-derived optical materials but also for gaining insight into its native structure within the cell wall. The luminescence properties of lignin vary across taxonomic species and extraction methods. Moreover, because lignin exhibits a broad molar mass distribution, its chromophores are likewise expected to span a wide molar mass range. In this study, milled wood lignins were isolated from four lignocellulosic sources representing different taxonomic groups: Japanese cedar (gymnosperm, softwood), Japanese beech (angiosperm, dicotyledonous hardwood), palm frond (angiosperm, monocotyledonous Arecaceae ), and corn cob (angiosperm, monocotyledonous Poaceae ). Next, the molar mass distributions of chromophores in these samples were analyzed using size-exclusion chromatography across various excitation and emission wavelengths to examine their fluorescence behavior in solution. The results revealed that, across all samples, the fluorescence-based molar mass distribution varied slightly with emission wavelength (e.g., 350 nm and 400 nm), even when the excitation wavelength was fixed at 320 nm. Furthermore, differences in distribution were observed at different excitation wavelengths (320, 350, and 400 nm), suggesting that lignin comprises a variety of chromophores, each associated with a distinct molar mass distribution.