Indigo Day!

Today I Learned:
Facts about dyeing jeans!

1) As of about ten years ago, about 3 BILLION jeans were sold annually, making jeans a rougly $66 billion/year industry. For scale, that's something like a fifth of the US military budget, or about nine times the budget of the NSF.

2) Indigo, the dye used to color jeans, has some very special properties. First off, it's gorgeous. But then, you already knew that*. What's really special is that it binds to cotton and other fibers without any covalent bonding. For reasons I don't really understand, this lets it dye the outside and *only* the outside of a thread, which is why it fades as the outside of the thread abrades away. The fading thing is very important.

Of course, there are plenty of dyes that don't covalently bind. What separates indigo from all the other non-covalent dyes is that it *is* persistent enough to stick to cotton even through heavy bleaching and extreme heat (i.e., the inside of a washing machine/dryer cycle). Most non-covalently binding dyes come right off in water, much less *bleach*.

* I mean, how could you not?

3) Ever seen an indigo plant? (actually there are several, but any one will do for these purposes.) The first thing you'll notice is that it isn't blue. At least, most of the time. See, the indigo plant actually stores a modified form of the molecular precursor to indigo, called indoxyl, in special vesicles in its cells. Indoxyl will spontaneously react with air to form indigo, but before it can, the cell caps its reactive group with a glycosyl group, which stabilizes it. When the indigo plant is stressed, for some reason it releases glycosylated indoxyl from its special vesicles, uncaps the glycosyl groups, and turns brilliant blue. You can induce this by spritzing an indigo plant with ethanol.