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Breaking the biomass barrier

Anyone who’s dealt with an uncooperative kid or combative employee is familiar with the concept of recalcitrance – stubbornness, disobedience and noncompliance.

It’s easy to see why scientists would apply the same term to lignocellulose. The woody material found in plant cell walls makes trees tough and stems stiff, but also locks up sugars, making it difficult to ferment them into ethanol fuel.

Atomic-detailed model of lignocellulose of softwoods. Based on experimental data on the structure of cellulose (brown) and lignin (cyan and red).

“Plants don’t want to be hydrolyzed and broken down into sugars. They develop these defense systems,” says Jeremy Smith, director of the Center for Molecular Biophysics (CMB), an Oak Ridge National Laboratory (ORNL)-University of Tennessee joint project. In other words, the plant materials that make up biomass for energy are recalcitrant.

Smith, who also is the first University of Tennessee (UT)-ORNL Governor’s Chair, and his fellow researchers are out to learn the atomic basis for this recalcitrance. With a 3.5 million processor-hour grant of computer time through DOE’s Innovative and Novel Computational Impact on Theory and Experiment (INCITE) program, Smith is building computational models of lignocellulose and testing how enzymes interact with it and each other. They’ll compare their models against neutron scattering experiments generated at ORNL’s Spallation Neutron Source (SNS).

The Oak Ridge CMB participates in the Department of Energy’s BioEnergy Science Center (BESC), an interdisciplinary coalition of experts from ORNL, UT and other universities, corporations and the National Renewable Energy Laboratory (NREL). The BESC mission is to achieve breakthroughs in biofuels from lignocellulosic biomass.

What they learn could help make cellulosic ethanol readily available and economically viable, helping the United States replace foreign oil with a renewable resource and curb greenhouse gas emissions. As it stands now, making ethanol from biomass like plant stalks or wood chips is difficult and expensive, requiring substantial energy and chemical inputs. Cellulosic ethanol production could generate less carbon dioxide and interfere less with food supplies, but it’s still easier and less expensive to make ethanol from corn and other crop-based feedstocks.

“We need to understand why (plant cell walls) are recalcitrant,” Smith says. “To do that you need to understand the structures of plant cell walls and what’s stopping them from being broken down by enzymes.”

Cellulose, the source of fermentable sugars in biomass, is locked inside some pretty tough stuff, as researched by CMB graduate student Benjamin Lindner and postdoctoral research fellow Loukas Petridis. Cellulose is contained in compact, partially crystalline fibrils that block enzymes. Polysaccharides and lignin cover the fibrils, presenting another barrier that enzymes must overcome. The lignin also may inhibit enzymes by attaching to their cellulose-binding components. In fact, removing lignin from biomass increases the cellulose-hydrolysis yield from 20 percent to 98 percent, the paper says.

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Thomas R. O'Donnell

Thomas R. O'Donnell is senior science writer at the Krell Institute and a frequent contributor to DEIXIS.

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Thomas R. O'Donnell

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