 Have you gone through conformational transitions? Today we're going to first look at kinetics, that is how fast do things happen, and then use that to understand why and when conformational transitions happen, in particular protein folding. But first we're going to need to look a little bit about what structures and conformations we actually have during the protein folding process. Remember when I introduced protein folding, and we talked about that rapid hydrophobic collapse. If we just throw a chain like this in back as in water, it's instantly going to collapse to something that's compact, where we have the hydrophobic residues on the inside and the hydrophilic ones on the surface, and I even call this the multi-globular. We're going to focus a little bit on this multi-globular and what it means, because on the one hand we have something completely stretched out, in the middle we have some sort of semi-defined multi-globular, and then on the other side we have the actual folded protein. So maybe we should start thinking about the reverse process. If I start with my real native protein, what does it take to disrupt that, to go either to this multi-globular or all the way out to some really stretched out coil entirely without structure? It turns out that there are two ways we can denature proteins. The obvious one is temperature. So if I just draw something as a function of temperature, going from say, I don't know, maybe zero to 80 degrees centigrade or so. At some point here, I'm going to start denaturing the protein, and here I have a native one. I'll leave that a sec until I define what that is. But there's also another axis here. So instead of temperature, I could use a chemical denaturant. Say, maybe I go in a dinomion. This looks strange, but it's pretty much just an arginine side chain. This is a carbon and then three NH2 groups. Look at the arginine side chain and then you will see why this is very similar to it. This is going to participate in lots of hydrogen bonds and even so-called salt bridges because again, this is an ion. Plus one, it's going to love to interact with something that's negatively charged in the protein. That will quickly disrupt all the hydrogen bonds and everything on the surface of a protein and destroy structure almost completely. In particular, if I add not just a millimolar or something, but one, two, three or four molars. So if I throw in a huge amount of this, say going up to four molar, guanidinium hydrochloride. At some point here, I'm going to start destroying the structure completely too. And you will just have to trust me that there is some sort of region here where we're native. And then there's going to be some band between here. And what happens here? Here, I'm going to have the multin globule and up here, I have the completely stretched out coil. So if I just heat the protein, well, I will gradually start disrupting it a bit and eventually it will lose its native role. It's no longer going to function, but it's not really, it's disrupted, but it hasn't really completely unfolded. It's not working, but it's some sort of collapsed shape here. And if I throw a ton of very strong ions of it, then I add some temperature ideally, then I will at some point completely disrupt the protein and it's just going to be small strings of amino acids left. So that's on the one hand, we want to start in the transition to the native state but also the transition between the coil and the multin globule. The problem is that I haven't shown you any multin globules. So the first thing we're going to need to do is try to look at some of these structures and what they are. The problem though is that I can't really crystallize it because it's not a well-defined structure like the native structure. So you're going to have to make do with some hand-waving examples here and some structures we've been able to determine with NMR. Let me show you.