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Biomaterials

A surprising portrait of wood’s molecular makeup

NMR structures suggest the polymers in softwood are arranged differently than people thought

by Celia Henry Arnaud
November 18, 2019

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Credit: Nat. Commun.
A proposed model of softwood shows elementary fibrils made of cellulose (light and dark blue) with bound xylan (orange) and galactoglucomannan (pink). These in turn form macrofibril bundles with lignin (green) at the surface. When free of fibrils, xylan and galactoglucomannan adopt different conformations.
Scheme showing a model of softwood biopolymers in which elementary fibrils made of cellulose with bound xylan and galactoglucomannon form macrofibril bundles with lignin at the surface.
Credit: Nat. Commun.
A proposed model of softwood shows elementary fibrils made of cellulose (light and dark blue) with bound xylan (orange) and galactoglucomannan (pink). These in turn form macrofibril bundles with lignin (green) at the surface. When free of fibrils, xylan and galactoglucomannan adopt different conformations.

Softwood from trees such as spruce and pine is used to make paper, building materials, fuel, and chemical feedstock. But even though humans have long relied on this renewable resource, biochemists still haven’t had a full picture of its molecular architecture.

A team led by the father–son duo of Ray Dupree, a physicist at the University of Warwick, and Paul Dupree, a plant biologist at the University of Cambridge, used solid-state two-dimensional nuclear magnetic resonance spectroscopy to look at the molecular architecture of softwood from spruce at an unprecedented level of detail (Nat. Commun. 2019, DOI: 10.1038/s41467-019-12979-9). They hope their new model will lead to improved methods for processing and recycling products made from softwood.

Softwood cell walls are made up of four biopolymers: the polysaccharides cellulose, galactoglucomannan, and xylan, and the phenolic polymer lignin. Previous research by others had shown that the cellulose forms partially crystalline microfibrils, but what the other polymers do was less clear.

“The best guess was that the galactoglucomannans were closely associated with the cellulose microfibrils, and the xylan and lignin lay somewhere outside,” says Michael Jarvis, an expert on wood polymers at the University of Glasgow. The researchers “turn that structure inside out by showing, in great detail, that the xylan chains are in a highly ordered association with cellulose, taking up a conformation that is like the cellulose chains themselves.”

For the new study, the Duprees and their coworkers used solid-state 13C NMR to analyze interactions between the various polymers in softwood.

They showed that xylan adopts a two-fold structure that allows it to fit in grooves on the surface of the cellulose fibril. In solution, it adopts a three-fold structure. In their new model, they posit elementary fibrils consisting of cellulose, xylan, and galactoglucomannan (GGM). These elementary fibrils come together in 30-nm bundles that are surrounded by lignin. They put the lignin on the outside because the NMR data told them that cellulose has fewer interactions with lignin than it does with xylan and GGM, Paul Dupree says. Within each elementary fibril, the crystal planes are parallel, but the researchers don’t know yet whether the individual fibrils are aligned or randomly oriented within the larger bundle. So far, they don’t have a model for the interaction between GGM and cellulose, Paul Dupree says.

The new study “clearly indicates that most of the xylan in softwood is associated with the cellulose fibrils,” says Lennart Salmén, a scientist at Research Institutes of Sweden Bioeconomy/Biorefinery & Energy, who was involved in previous structural studies. “The structures proposed in earlier models were mostly due to the inability of the techniques used to detect the response from the xylan in connection with the cellulose.”

“The new model of the arrangement of the wood polymers is clearly an improvement to our understanding of structure–property relationships of the wood cell wall,” Salmén says. Jarvis agrees, calling the work “a real landmark in our understanding of the molecular structure of coniferous wood.”

The Duprees plan to further improve the model with factors such as GGM–cellulose interactions and the role of water. They hope that better molecular understanding of softwood will lead to improved paper processing and recycling methods.

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