Molecular Architecture of Softwood Revealed by Solid-State NMR

The molecular arrangement and interactions of lignin and polysaccharides in plant secondary cell walls are generally poorly understood due to technical difficulties of analysing this complex material which is made primarily out of the polysaccharides cellulose, xylan and galactoglucomannan and the polyphenolic polymer lignin.

The results of a multidimensional solid-state NMR show that lignin is associated with both xylan, galactoglucomannan and surface chains of cellulose. Xylan and Galactoglucomannan bind to the same cellulose microfibrils. Some lignin is close to hemicelluloses and cellulose. Whereas covalent linkages may mediate such proximity between the two, this feature cannot be firmly established are the sensitivity of these solid-state NMR experiments is not yet high enough to identify such rare events.

Possible models of the molecular architecture of softwood. On the right, binding of both xylan and some GGM to the hydrophilic surface can change domain 2 to domain 1. Lignin is shown mostly associated with itself, but is close to GGM, xylan and domain 2 cellulose. The cellulose microfibrils are taken to have 18 glucan chains to match the measured cellulose domain 1 to domain 2 ratio upon xylan binding.
Model of spruce cell wall microfibril. Groups of cellulose microfibrils with bound GGM and xylan form macro fibrils in spruce cell walls. In addition to cellulose-bound xylan and GGM macro fibrils may contain some three-fold xylan and matrix GGM. Lignin is localised to the surface of the polysaccharide core of the macro fibrils and interacts predominantly with GGM, xylan and cellulose domain 2.