 fibrous proteins are large building blocks that are cells used to create extended structures in our body. They're going to consist of lots of things such as parts of your skin, your bone, hair, teeth and on the one hand you might not think of these as biological material but they are definitely biological in the sense that our bodies have to be able to build them one way or another. They also need to be rigid, they need to be macroscopic in size compared to most other proteins. You can see a bone, right? You can't see a single hemoglobin molecule and this makes it a bit different. For once you there is no way you could have an amino acid sequence that is so long that way they would create a protein that would be a millimeter long. That would kind of be like the dimension of DNA. There is no way that can happen. So to build something that reaches macroscopic dimensions we're going to need to repeat stuff. One building block and then the same building block and the same building block again and that's a characteristic of fibrous proteins. Let me start with an example to show how we repeat building blocks. So in this case the first fibrous protein we're going to look at is a beta sheet and beta sheets you might remember they were pleated with amino acids alternatively pointing up and down. So I'm going to draw such an amino acid and let's start with serine and then I have glycine and then alanine and then glycine again not a whole lot of imagination here and then alanine again and then glycine again. And then we repeat this many times. Typically you might have it eight or ten times but that's just one unit. After that you might have a few other amino acids and then this eight mere repeats again. So in total we're going to have possibly tens of thousands of units like this. It goes on forever. Now at first sight this might look really boring but the point says we repeat it. It's actually going to become a very long sheet right? And these sheets turn into silk. Silk is a very special substance that you've likely felt and it's the specific amino acid composition here that gives silk these properties. So for once you have lots of glycine here. Do you remember that glycine is a very small and flexible amino acid? That helps create this shiny shiny surface and everything that it's not particularly hard or anything. We have alternating properties of alanine and glycine here that is intimately related to this property of beta sheets being pleated. So if you look at this sheet here we're going to have on one side here we're going to have serine alanine alanine. Serine alanine alanine. Serine alanine alanine. Keep repeating. But on this side we're only going to have glycine. Glycine, glycine, glycine, glycine, glycine, glycine. If we do that in some sort of stack if you now put several of these sheets on top you're going to have alternating layer here where the blue layer is going to be very thin that's just glycine and then a slightly thicker layer. Serines and alanines facing each other and there will even be some hydrogen bonds evolving the serines here. So this is going to create a fairly dense thick nice layer and while you might not think of it as a hill this is going to be a super strong fiber. So beta sheets actual silk is roughly 80% of the structure. There is some other proteins make sense with that we're not going to care so much about. You have an alternating anti-parallel first of the sheets themselves are anti-parallel but then you also have an alternating up-down orientation of the sheets here. So that's where you get these characteristic distances between the sheets. This fiber is so strong that the tensile strength per weight at least equals steel. The reason why we can still pull the fibers apart occasionally is of course that they're macroscopically thin right? They're tiny, they're micro-sized. But if we now take many of these fibers they can become a very strong thread. The other neat thing with this thread is that it's biodegradable. So what if you would like to sew something but it should be degraded by a body? What would you need that? Well say a suture if you're a surgeon right? And there are several other cases where you might like a particular property that should be either compatible with the body or simply that you would like to engineer properties on molecular level which we can't really do with steel. So this is certainly done with silk not necessarily silkworm but one material that is very popular to try to buy engineer now is spider silk. So I won't have time to go through all the details of the generation of spider silk but even in Sweden there are at least two research groups, colleagues of mine, that are working on producing silk but then by artificial means. If you ever go out and buy a bottle of shampoo or so you might occasionally see that it contains silk protein and I guess the vendors do that to make it look expensive. It's not expensive. That protein has never been anywhere near a silkworm or anything. All it means is that that particular bottle of shampoo contains bioengineered beta sheets that they've just produced in bacteria or something that happens to have roughly this composition and again that might help create some glossiness in your hair. Maybe I don't. I'm not that much into cosmetics. But this is the first example of a super simple building block that then is repeated and that gives a large scale macroscopically sized material because again you can see the silk, its particular properties. Pure beta sheets, plain and simple.