Obtaining pure spectra of hemicellulose and cellulose from poplar cell wall Raman imaging data

A comprehensive understanding of the structural and chemical nature of plant cell walls is important from both the perspectives of plant biotechnology and of commercial utilization. The Raman imaging technique is a preferred solution for its ability to offer spatial and spectral information simultaneously. However, the exact spectra of hemicellulose and cellulose are difficult to directly discern from each other due to the strong spectral overlap. In the present study, we report for the first time that the spectra of hemicellulose and cellulose in poplar cell wall Raman imaging can be discriminated by using multivariate analysis. The semi-quantitative concentrations of polysaccharides are identified based on the corresponding Raman images. Results show that cellulose is mostly concentrated in the secondary wall of poplar fibres, whilst the distribution of hemicellulose is almost uniform throughout the cell wall of fibres except for a higher concentration found in the S1 and the outer S2 layer. The xylem ray and the vessel have relatively high hemicellulose concentrations which is comparable to the outer S2 layer of fibres, but the cellulose concentration is relatively low in these two cell types. This work not only contributes to broadening our knowledge about the distribution of polysaccharides in plant cell walls, but also provides a potential strategy to trace their dynamic changes during the bioconversion at a micro-level.