Over the past two years I’ve been heavily involved in the chemistry of wood. I’ve developed numerous patent-pending technologies which either use wood as a starting chemical or somehow augment it’s properties. One of the biggest difficulties I’ve encountered when using wood as a reaction component is that the most valuable part of wood, chemically speaking, can be hard to separate from the “trash”. When people think about the valuable fractions of wood, they’re usually thinking about cellulose. Cellulose is a polymer (repeating molecule, linked end to end) of glucose units. Humans don’t have the necessary enzyme to break down the cellulose polymer bonds, although some insects (including termites) can ingest cellulose as food. Several chemists, including myself, have developed promising methods of transforming cellulose into other chemicals that are valuable – ethanol, or other biofuels, for example.
The difficulty, as I mentioned, is that cellulose and hemicellulose (the two valuable components of wood) are mixed together with something called lignin. Lignin is a heavily crosslinked polymer that gives stems and wood grain their strength and rigidity. Hardwoods contain about 30% lignin, by weight. Compared to cellulose, lignin molecules are an absolute mess. They have random structures with no particular pattern or design. Anyone wanting to access the cellulose for a chemical process first has to find a way of dealing with the lignin. While there are techniques to do so – mixing the wood pulp with acid and sulfite, for example, which is one of the steps in paper production – the result is a black, sludgy material called lignosulfonate which is extremely difficult to process into anything useful. Normally, it’s just burned for it’s energy content.
While I’ve developed several useful methods of using these lignosulfonates for something besides fuel, it’s still readily apparent to me that wood is a very inconvenient chemical starting material. While the cellulose is mostly regular and we can design reaction sequences for it, trying to get the cellulose away from the lignin can be difficult. That’s why I was interested to read an article in probably the top chemistry journal in the world, Science, which outlined a study of a brown fungus – more precisely, it was a study of the fungus genome. There were a lot of researchers involved with this project, and the article outlined how the fungus in question (Serpula lacrymans, or “creeping tears”) uses it’s enzymes to perform a surgical strike against cellulose. Enzymes are Nature’s catalysts, and as much progress as we chemists like to think we’ve made over the last two hundred years, we’ll never approach the sophistication and utility present in Nature.
The article revealed that the dry rot fungus does so much damage so quickly because it doesn’t have to expend any extra energy or effort towards breaking down woods fractions. Unlike chemists, who have to unravel the cellulose from around the lignin (or blast the whole mass with chemicals, which can be difficult to clean up), the fungus enzymes selectively attack the cellulose present in wood. The glucose polymer unravels, which provides food for the fungus. The remaining 25-30% lignin component isn’t strong enough by itself to support the wood structure; it’s more of a scaffold than a weight-supporting material. As a result, the hollowed-out wood collapses under it’s own weight. It’s a lesson which chemists such as myself would do well to take to heart, in that we don’t need to design stronger reagents or chemicals, we need to design smarter ones. This genome report will no doubt lead scientists in the direction of smarter wood technologies. It may even be possible to use the fungus to our own ends, so that production of biofuels (which also require cellulose) can be performed without having to worry about the lignin. The Science article will definitely be referenced heavily in the future, and I’m excited to see what technologies will result.
The source of this article can be found at:
Eastwood, D. “The plant cell wall decomposing machinery underlies the functional diversity of forest fungi”. Science 2011, 333, 762-765.