In my last post on lignin, I introduced the hypothesis that it’s the movement of cellulose that causes monolignol toroidosomes to knot into double-helical lattices. I also introduced my guess for the steps required for the knotting. I also introduced the concept of cellulose being a catalyst for this process. In today’s post I’ll try to flesh out these ideas and see where I get.
To begin with, let’s take the step-by-step image from the last post and add three curves into it to depict the growing cellulose fibers, like this:
Of course, this image is a cartoon version of knotting. It probably isn’t so that there would be a gradual progression from unknotted toroidosomes to the final hexagonal double-helical lattice along a single cellulose fiber. Rather, this process seems to be likely to progress over time, where the whipping motion of probably more than one cellulose fiber within the hole of the toroidosomes expands the toroidal helical loop.
Then, at a tipping point the helices of neighboring toroidosomes are so deeply interlaced that the whipping motion of the cellulose fiber no longer expands the toroidosomes, but rather causes the hydrogen bonds between monolignols to open, allowing the toroidosomes to knot. Once all the loops have knotted, then the equilibrium state consists of double-helical nanotubules that are hydrogen bonded to all four nanotubules with which it is at contact.
At least in wood, there isn’t just one cellulose fiber of 2-3 nm diameter within the toroidosomes hole. Like this paper shows, the hemicellulose-lignin module packs several of the ‘branches’ of cellulose.
So, are plants ‘woody’ immediately after this process? Not yet. Next, the monolignols need to polymerize. Before this, the structure is still held in place with hydrogen bonding, and the interlinked helices in the double-helix not even with that. I talked about this polymerization in my previous post, so I’ll just show you an image from it.
Once the structure has lignified, then the double-helices are covalently linked with each other. Even then, the polymers aren’t infinitely long. In fact, I recall that they are more like 20 to 30 monolignols long, which in the greater scheme of things is almost nothing. But this allows the lignin polymers to be well and truly interlinked in a manner that it is exceedingly difficult to pry them apart anymore.
But of course, we chemists and engineers have learned to do that. The most common means for this is the kraft process, where wood is first cut into chips, then ‘cooked’ in a rather potent chemical solution that significantly alters the chemical nature of lignin. This ‘cooking’ is required to detach lignin from cellulose, so you can make pulp, which in turn is used to make paper and carboard and other products.
However, this chemical alteration doesn’t change the supramolecular shape of the lignin nanotubule. That’s probably because the process is optimized so that once lignin can be separated from cellulose, you don’t need to break down the structure further. Whether more cooking would destroy the nanotubular structure, I don’t really know.
Anyhow, the hexagonal lattice is seen when kraft lignin is self-assembled in just the right conditions, but you won’t see individual nanotubules in this hexagonal lattice. For this, you would need to have covalent bonds to keep each nanotubule at a 60-degree angle to their neighbor. And the role of kraft pulping is to break these covalent bonds.
I’ll these musing for now, but I’ll sure be posting on the structure of lignin until I have a manuscript to submit.
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