Influence of the Molecular Motifs of Mannan and Xylan Populations on their Recalcitrance and Organization in Spruce Softwoods
Softwood from conifers constitutes one of the main terrestrial renewable resources for the production of bio-based materials and platform chemicals. Lignocellulose from softwoods has a distinct molecular composition compared to other plant biomass sources, where acetylated galactoglucomannan is the main hemicellulose with minor amounts of arabinoglucuronoxylan. Here, we reveal the presence of mannan and xylan populations in spruce softwoods with distinct molecular features based on their extractability using sequential hydrothermal treatment by subcritical water without previous delignification. An accessible acetylated mannan population has been identified with simple profiles of glucosyl and galactosyl motifs and without the existence of a regular acetylation pattern. The xylan populations are extracted at intermediate times, and they exhibit the presence of major and minor regular intramolecular domains with different relative abundance based on extractability. Finally, a recalcitrant mannan population with complex glucosylation and galactosylation profiles was identified at longer extraction times. Molecular dynamic simulations revealed that the presence of consecutive mannose units in the backbone prevents the tight association with cellulose surfaces, which may explain the different extractability of the two isolated mannan populations. The combination of sequential hydrothermal treatment, comprehensive carbohydrate sequencing and molecular dynamic simulations offer new insights on the distinct features of the mannan and xylan populations in softwoods, and their putative organization in the lignocellulosic matrix.