John Ralph PhD’82 talks with the easy, garrulous rhythms of his native New Zealand, and often seems amiably close to the edge of laughter.
So he was inclined toward amusement last year when he discovered that some portion of the Internet had misunderstood his latest research. Ralph—a CALS biochemist with joint appointments in biochemistry and biological systems engineering—had just unveiled a way to tweak the lignin that helps give plants their backbone. A kind of a natural plastic or binder, lignin gets in the way of some industrial processes, and Ralph’s team had cracked a complicated puzzle of genetics and chemistry to address the problem. They call it zip-lignin, because the modified lignin comes apart—roughly—like a zipper.
One writer at an influential publication called it “self-destructing” lignin. Not a bad turn of phrase—but not exactly accurate, either. For a geeky science story the news spread far, and by the time it had spread across the Internet, a random blogger could be found complaining about the dangers of walking through forests full of detonating trees.
Turning the misunderstanding into a teachable moment, Ralph went image surfing, and his standard KeyNote talk now contains a picture of a man puzzling over the shattered remains of a tree. “Oh noooo!” the caption reads. “I’ll be peacefully walking in a national park and these dang GM trees are going to be exploding all around me!”
That’s obviously a crazy scenario. But if the technology works as Ralph predicts, the potential changes to biofuels and paper production could rewrite the economics of these industries, and in the process lead to an entirely new natural chemical sector.
“When we talk to people in the biofuels industry, what we are hearing is that creating value from lignin could be game-changing,” says Timothy Donohue, a CALS professor of bacteriology and director of the UW–Madison-based Great Lakes Bioenergy Research Center, where Ralph has a lab. “It could be catalytic.”
After cellulose, lignin is the most abundant organic compound on the planet. Lignin surrounds and shapes our entire lives. Most of us have no idea—yet we are the constant beneficiaries of its strength and binding power.
When plants are growing, it’s the stiffening of the cell wall that creates their visible architecture. Carbohydrate polymers—primarily cellulose and hemicelluloses—and a small amount of protein make up a sort of scaffolding for the construction of plant cell walls. And lignin is the glue, surrounding and encasing this fibrous matrix with a durable and water-resistant polymer—almost like plastic. Some liken lignin to the resin in fiberglass.
Without lignin, the pine cannot soar into the sky, and the woody herb soon succumbs to rot. Found primarily in land plants, a form of lignin has been identified in seaweed, suggesting deep evolutionary origins as much as a billion years ago.
“Lignin is a funny thing,” says Ralph, who was first introduced to lignin chemistry as a young student during a holiday internship at New Zealand’s Forest Research Institute. “People who get into it for a little bit end up staying there the rest of their lives.”
The fascination is born, in part, from its unique chemistry. Enzymes, proteins that catalyze reactions, orchestrate the assembly of complex cell wall carbohydrates from building blocks like xylose and glucose. The types of enzymes present in cells therefore determine the composition of the wall.
Lignin is more enigmatic, says Ralph. Although its parts (called monomers) are assembled using enzymes, the polymerization of these parts into lignin does not require enzymes but instead relies on just the chemistry of the monomers and their radical coupling reactions. “It’s combinatorial, and so you make a polymer in which no two molecules are the same, perhaps anywhere in the whole plant,” says Ralph.
This flexible construction is at the heart of lignin’s toughness, but it’s also a major obstacle for the production of paper and biofuels. Both industries need the high-value carbohydrates, especially the cellulose fraction. And both have to peel away the lignin to get to the treasure inside. A combination of heat, pressure, and caustic soda is standard procedure for liberating cellulose to make paper; bleach removes the remaining lignin. In the biofuels industry, a heat and acid or alkaline treatment is often used to crack the lignin so that it is easier to produce the required simple sugars from cellulose. Leftover lignin is typically burned.
The economic cost of these treatments alone is significant, and lignin pretreatment is at the heart of many of the more egregious environmental costs of paper. On the biofuels side, lowering treatment costs to liberate carbohydrates from lignin could change the very economics of biofuels. In these large-scale, industrial processes, saving a percentage point or two is often worthwhile, but the Holy Grail is a quantum jump.
“Because it’s made this way”—Ralph jams his hands together, crazy-wise, fingers twisted together into a dramatic representation of lignin polymerization—“there is no chemistry or biology that takes it apart in an exquisite way,” he says. “We actually stepped back and thought: How would we like to design lignin? If we could introduce easily cleavable bonds into the backbone, we could break it like a hot knife through butter. How much can you actually mess with this chemistry before the tree falls down?”
Ralph’s team had their eureka moment more than 15 years ago, and have been trying to bring it to life ever since.
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