 So if you repeat those units, two by two by two, you will form larger and larger structures. And these structures will look something like this. You're going to start from something that's the size of a few angstroms or so. You will go up to nanometers, micrometers, and eventually submillimeters. And this is the structure of hair fibers. Not just human hair, but wool anything in nature, pretty much. And in fact, in the honor of today's class, I'm wearing an Alpha Helix sweater. It doesn't say that when you buy it, it says wool, but wool is like probably 75% Alpha Helix. There are a couple of interesting concepts with this. First, this is how the structure keratin was actually discovered originally. Bill Astbury was working at the textile institute in the UK in the 1930s. And they were after a lot of work able to extract the protein corresponding to the components of wool. They were able to crystallize it. And he was able to see that there is some sort of repeating unit just over five angstroms. I think he said 5.1 angstroms in period. And it appears to somehow be a spiral shape. And based on that, he said that I'm going to call this Alpha keratin. And he assumed that it's some sort of Helix. And then he guessed that the reason why wool in particular is that you can stretch it a bit is that if we're stretching it, this Helix is going to act like a spring. We can distort it. And then it's going to be a beta keratin Helix. And then when I relax it again, it's going to go back to the Alpha form. Now, that wasn't quite right. The protein is called Alpha keratin. That's quite right. There are helices in it, but we don't stretch those helices. The reason why we can stretch, you can't, we can stretch an individual hair fiber, right? The reason why these things are reasonably flexible is that the fibers go in the zigzag and we stretch out the fibers. We don't stretch the individual fiber. There is two interesting applications here. First, if you ever go to the hairdresser and get a permanent wave, those bonds, the disulfide bridges, will be reduced by a chemical. Now you're breaking the bonds between adjacent helices. And that makes the hair much floppier. Well, it's not so much the floppiness, but you're breaking the inherent structure of the hair. Now you can form it into whatever shape you want almost. And then you're adding a second chemical, both to neutralize the first and re-oxidize those self-hybridges in new shape, in the current shape. And that will stabilize the particular form of the hair. So for us, you get it curly. And it's going to stay that way for weeks, or at least until it grows out again. Second, if you're wearing any type of wool fiber or so, you should be careful how you wash it, because this is literally a protein sweater. Modern detergent is super efficient, and we can wash clothes at very low temperatures, even 30 degrees centigrade or so. The reason for that is that we're adding enzymes, literally proteins, or occasionally artificial enzymes, so-called proteases. So these proteases are enzymes that would break down proteins, which is good, because a lot of dirt might be either food or, well, remnants from our skin or some things, so it's good to break down proteins. We don't want it. There are certainly no proteins in cotton, and there are no proteins in polyester or any of the normal materials we normally use for clothes. But the problem is, if your piece of clothes is a protein itself, it's going to start attacking the clothes. And alpha helices are fairly stable, so it's certainly not going to break it down over one or a few washes. But over the long run, if you keep adding detergent, the detergent will wear out the protein, including the alpha helices. And that's why if you have natural fibers in your clothes, you should use a detergent without enzymes, literally, to maintain the protein. Based on how fast hair grows, roughly half a millimeter per day, and this spacing of each turn in an alpha helix, now you should be able to calculate how many turns in an alpha helix are added each second. But I leave that as an exercise for you. It will tell you whether you were right in last lecture predicting how fast helices work.