Unlocking the structure of lignin, and feasible pathways to cellulosic conversion

June 16, 2011 |

Research from ORNL reveals new details of bioenergy’s most daunting technical barrier

“You can make anything out of lignin, except money.” – old industry saying

Lignin, for those newer to the biofuels world, is a complex chemical compound that makes veggies, plants and wood firm, confers resistance to pests, and provides the skeletal strength to enable plants to reach up to the sun for energy.

So that’s a good thing.

If you know your Star Wars, lignin is sort of like the Force. “It surrounds us, penetrates us, and binds the galaxy together,” as Obi-Wan Kenobi observed.

One more thing: it’s highly resistant to biofuels-related enzymes, critters and magic bugs that seek to unlock the higher value in the sugars. Lignin is the single biggest technical bio-processing barrier to cellulosic biofuels. It’s exceedingly difficult to separate lignin from cellulose, and costly. After separation, lignin remains a low-value, unloved byproduct that is typically burned to generate power.

So that’s a bad thing.

As Catullus wrote in Odi et Amo, so it is with lignin, “I hate and I love, how can I explain this contradiction? I can only feel it, and I am in agony.”

Of course, it makes you just want to sit down and talk to a plant. “You know,” a scientist might say to a bundle of switchgrass, with a voice dripping with honey, “we’d plant a lot more of you, if you could just ease back on the lignin.”

Vive La Resistance

The quality that lignin has is recalcitrance – resistance to invasive change. We see that quality in so many aspects of the conversion of the fossil oil-based society to renewables. There’s recalcitrance, or resistance,  in  making flex-fuel cars, blender pumps, pipelines, and tax structures. We’re a recalcitrant species – but nothing breaks down human resistance faster than “everyday low prices” –  and that’s where cellulosic recalcitrance comes in.

When people tell you that “cellulosic biofuels are five years away” (which, thankfully, they aren’t any more), that is a function of high enzyme costs. In turn, that’s a function of high enzyme loads, which in turn reflects how pesky all that lignin is in resisting the new-fangled science that attempts to convert plants into fuels.

Know thy enemy: A breakthrough in understanding lignin

A folded protein

One thing we do known about lignin, is that it forms clumps and lumps. Persnickety scientists call them aggregates. When enzymes are used to release plant sugars necessary for ethanol production, the lignin aggregates bind to the enzymes and reduce the efficiency of the conversion.

Barriers to solving that problem? We don’t know as much as we need to about what lignin looks like at the molecular level. It’s like hunting down a fugitive without a mug shot.

Recently a team down at the Oak Ridge National Laboratory led by Jeremy Smith combined simulations on ORNL’s Jaguar supercomputer, and neutron scattering at the lab’s High Flux Isotope Reactor, to resolve lignin’s structure at scales ranging from 1 to 1,000 angstroms. Smith’s project is the first to combine the two methods in biofuel research. Their work was published this week at Physorg.com.

Now, 1 angstrom is pretty small. 500,000 angstroms is the width of a human hair. An atom is between 1 and 5 angstroms.

The Complex Origami of Lignin

Their finding? Lignin is highly folded. Roll a ball across a bed, then roll a ball across an unmade bed, and you’ll see the problem. The highly folded, complex structure of lignin creates more opportunities to capture the passing enzymes than a smooth surface would.

Proteins themselves are highly folded, and enzymes are proteins. The effect is a little like velcro, and that causes enzymes to slow down (when in fact their general function is to speed up chemical reactions by breaking up molecules, and reforming them).

An improved understanding of the lignin aggregates will aid scientists, say the ORNL team, in efforts to design a more effective pretreatment process, which in turn could lower the cost of biofuels. That’s highly welcome.

Even more opportune than finding new ways to separate lignin from cellulose will be finding new ways to break down the lignin molecules themselves, more effectively. Lignin is one of the main contributors to the formation of soil humus (when it breaks down in the soil), the primary constituent of healthy topsoil. Engineering a more effective breakdown may well contribute to restoring soil.

It’s a challenge – lignin is known for its near-chaotic variety – virtually no two lumps are the same, with a wide variety of complex aromatic rings and structures.

But, in the nearer term, using all those hydrocarbons for something more valuable than burning them in ovens to generate steam power – well, that’s an area of real opportunity.

More on lignin

For more on opportunities and work in overcoming (plant) recalcitrance, try our look at Oak Ridge’s work on the genetic structure of Z.mobilis,  or work by the Samuel Roberts Noble foundation, Georgia Tech and ORNL on switchgrass, or ArborGen’s low-lignin eucalyptus trees.

And some brand new research published in Biochemistry looks at a bacterium that is showing real promise in speeding up lignin breakdown.

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