Secondary cell wall polymers studied by Confocal Raman microscopy: Spatial distribution, orientation and molecular deformation

Confocal Raman microscopy was used for chemical imaging of wood cell wall polymers and to follow molecular changes during tensile deformation. Spectral maps were acquired from cross-sections of poplar wood and images calculated by integrating the intensity of characteristic spectral bands. This enabled direct visualisation of the spatial variation of the lignin content without any chemical treatment or staining of the cell wall. A higher lignin content was visualised in the cell corners, the compound middle lamella, and the secondary cell wall of vessels than in the fibres. The S1 was distinguished from the S2 by integrating over the band at 1097 cm-1, because the intensity of this vibration is sensitive to the orientation of the cellulose molecule. The position of this band was shifted towards shorter wavenumbers during straining of wet tangential sections, demonstrating that the cellulose molecule was subjected to a deformation. The band shift was followed during the tensile test and a good trend and correlation with strain and stress were observed. Investigating tissue types with different properties and cell wall assemblies will help to reveal the polymer composition and orientation non-destructively with a high spatial resolution. By investigating structural changes during tensile straining, we aim at understanding the different stress-strain behaviour and the molecular mechanistic phenomena involved