Supercomputer probes depths of biofuel's biggest barrier

Ask a biofuel researcher to name the single greatest technical barrier to cost-effective ethanol, and you're likely to receive a one-word response: lignin.

Cellulosic ethanol—fuel derived from woody plants and waste biomass—has the potential to become an affordable, renewable transportation fuel that rivals gasoline, but lignin, one of the most ubiquitous components of the plant cell wall, gets in the way.

In nature, the resilient lignin polymer helps provide the scaffolding for plants, reinforcing slender cellulosic fibers—the primary raw ingredient of cellulosic ethanol—and serving as a protective barrier against disease and predators. Lignin's protective characteristics persist during biofuel processing, where it's a big hindrance, surviving expensive pretreatments designed to remove it and blocking enzymes from breaking down cellulose into simple sugars for fermentation into bioethanol.

To better understand exactly how lignin persists, researchers at the US Department of Energy's (DOE's) Oak Ridge National Laboratory (ORNL) created one of the largest biomolecular simulations to date—a 23.7-million atom system representing pretreated biomass (cellulose and lignin) in the presence of enzymes. The size of the simulation required Titan, the flagship supercomputer at the Oak Ridge Leadership Computing Facility (OLCF), a DOE Office of Science User Facility, to track and analyze the interaction of millions of atoms.

The research, led by Jeremy Smith, a Governor's Chair at the University of Tennessee (UT) and director of the UT-ORNL Center for Molecular Biophysics, revealed in atomistic detail why lignin is such a problem: Not only does it bind to cellulose in the preferred locations sought by enzymes, but lignin also attracts and occupies the cellulose-binding domain of the enzymes themselves.