Researchers at the U.S. Department of Energy’s Los Alamos
National Laboratory and Great
Lakes Bioenergy Research Center
have found a potential key for unlocking the energy potential from non-edible
biomass materials such as corn leaves and stalks, or switch grass.
In a paper appearing in the Journal
of the American Chemical Society, Los Alamos researchers S. Gnanakaran,
Giovanni Bellesia, and Paul Langan join Shishir Chundawat and Bruce Dale of
Michigan State University, and collaborators from the Great Lakes Bioenergy
Research Center in describing a potential pretreatment method that can make
plant cellulose five times more digestible by enzymes that convert it into ethanol,
a useful biofuel.
Biomass is a desirable renewable energy source because
fermentable sugars within the cellulose network of plant cells can be extracted
with enzymes and then converted into ethanol—if only it were so simple. One of
the key difficulties in creating biofuels from plant matter is that the
cellulose tends to orient itself into a sheet-like network of highly ordered,
densely packed molecules. These sheets stack upon themselves and bond together
very tightly due to interactions between hydrogen atoms—somewhat like sheets of
chicken wire stacked together and secured by loops of bailing wire. This
stacking and bonding arrangement prevents enzymes from directly attacking most
of the individual cellulose molecules and isolating the sugar chains within
them.
Currently, ethanol can only be extracted in usable
quantities if the biomass is pretreated with costly, potentially toxic
chemicals in an energy-intensive process. Now, however, the research team has
discovered a way to develop potentially cost-effective pretreatment methods
that could make biomass an economically viable contender in the biofuels arena.
Using recent experimental data provided by their journal
collaborators, Gnanakaran and his Los Alamos colleagues
used state-of-the-art computational methods and molecular modeling to examine
how cellulose changes structurally into an intermediate form that can be
enzymatically attacked when pretreated with ammonia.
“Our modeling showed, and the experimental evidence
confirmed, that the pretreatment reduced the strength of hydrogen bonds in the
cellulosic network,” says Gnanakaran. It was as if the bailing wire in the
bound chicken-wire analogy had been removed and replaced more loosely with
thread. This, in turn, significantly reduced the tightness of the cellulose
network and left it more vulnerable to conversion into sugar by fungi-derived
cellulolytic enzymes.
The end result is a potentially less costly and less energy
intensive pretreatment regimen that makes the cellulose five times easier to
attack.
“This work helps address some of the potential cost
barriers related to using biomass for biofuels,” Gnanakaran says.
