A father-and-son team of researchers examined the two most common polymers on Earth, xylan and cellulose, which are found in the cell walls of materials like straw and wood. While scientists knew the two molecules play major roles in determining the strength of materials, and how easily they can be digested, they were unsure how the two molecules stick together.
Xylan is a long, winding polymer with “decorations” of other molecules and sugars attached, while cellulose is a thick, rod-like molecule. The physical differences have puzzled researchers, who were unsure how the polymers combine in cell walls.
“We knew the answer must be elegant and simple. And, in fact, it was,” said lead author Paul Dupree. “What we found was that cellulose induces xylan to untwist itself and straighten out, allowing it to attach itself to the cellulose molecule. It then acts as a kind of ‘glue’ that can protect cellulose or bind the molecules together, making very strong structures.”
Dupree, a professor from the biochemistry department at the University of Cambridge, uncovered this phenomenon several years ago, after studying Arabidopsis – a small flowering plant related to mustard and cabbage. He and his team found that the decorations on xylan can only occur on alternate sugar molecules within the polymer – essentially meaning they only appear on one side.
This unexpected discovery led the team to analyze other plants in Cambridge’s botanic garden; the phenomenon appeared to occur across all plants, suggesting that it evolved in ancient times – and must be important.
The team used an imaging technique known as solid state nuclear magnetic resonance (ssNMR), a process based on the same physics as hospital MRI scanners, but capable of revealing structure at the nanoscale. However, in order to image carbon, the technique requires carbon-13, a heavy isotope of the element. This required Dupree to grow the plants in an atmosphere enriched with a special form of carbon dioxide – carbon-13 dioxide.
“By studying these molecules, which are over 10,000 times narrower than the width of a human hair, we could see for the first time how cellulose and xylan slot together and why this makes for such strong cell walls,” said co-author Ray Dupree, Paul’s father.
Paul believes understanding how xylan and cellulose fit together could improve energy efficiency in industries ranging from agriculture to biofuels.
“One of the biggest barriers to ‘digesting’ plants – whether that’s for use as biofuels or as animal feed, for example – has been breaking down the tough cellular walls,” he said. “Take paper production – enormous amounts of energy are required for this process. A better understanding of the relationship between cellulose and xylan could help us vastly reduce the amount of energy required for such processes.”
The two molecules could also help scientists create stronger materials, according to the study. The United Kingdom already has plans to build more sustainable houses using wood, while Paul is involved in efforts to determine whether buildings as large as skyscrapers could be built using modified wood.
Their research was published Wednesday in the journal Nature Communications.