 So this means we need to look closer at the amino acid side chains and in particular their conformations. I've already talked about these bonds to you. And in the first lecture I think it was Elgemans and this polymer versus biochemistry conventions. It's going to be pretty rare that I talk about specific angles, but a particular view of physicists, this is worth remembering. You hopefully also remember that I told you that that peptide bond was special, that it was kind of a double bond in nature. And given this double bond nature that you have both the N and the carbon effectively wind up having each plane here that you see drawn on my left, is effectively six atoms in a plane. And that's because we have this N, both the N H group and the CO group forming a long rigid plane. And then you have the next peptide bond that's also six atoms stuck in a plane. That's actually very nice because it means that we have much fewer degrees of freedom in this chain than you might think. You only have basically this phi bond which is rotations around the bond just before the C alpha in each residue. And then the psi bond which is the rotation after each bond in the residue. And technically the very first phi bond in a long chain has no effect and the very last psi bond in a long chain has no effect. Which of course also means that if you just have one amino acid that is both first and last in the chain, rotations around these bonds won't really play a role. But in large chains they do. That's what led to those Rame-Shanton diagrams. The first time I showed you, I actually showed one that's a function of energy. But when starting out it was, you can take a molecule building block like this and just check when are they clashing or not. And if you do that, it's gonna turn out that there are some regions here that are much more advantageous than others. In fact, this is not based on a sitting with the molecular building block but statistics taken from modern structures of proteins. So there are in this phi on the x-axis and psi on the y-axis, there are only certain combinations that are allowed, relatively few of them, right? They're kind of just two or three large regions here. We're gonna need nomenclature for a few different bonds here that I'll show you. Let me draw you the site in here. We have n that is bonded to a previous chain. Then we have the alpha carbon and then we have the carbon and then we have the next chain there. And then we have the R group here. But the R group here could be more than one atom, right? So that there could be something else that I will call, let's say R1 and R2, say that they're carbons. This was the bond before the alpha. That's we call phi and the bond after the alpha is called psi. Let me draw the peptide bond for you too because that's gonna turn out to be useful. It's kind of convenient. That is a bond even though it's normal planar. Let's call that something. And we typically call that omega. It's planar and in virtually all cases, this is gonna be a transplant but for a few amino acids, proline in particular, it can occasionally rotate around and be cis. So it's pretty gonna be very rare that we look at the omega bond but if you see the expression, you will know that it is. All these other bonds are irrelevant but the side chain can move and it's convenient to have bonds for the side chain too. We typically call them the Greek letter chi one and then the second bond is chi two, et cetera. You can probably guess what they're called. It's gonna be chi three, chi four. Phi and psi are gonna determine the degrees of freedom along the entire chain here, right? If I show that to you here, we have a long chain. So we have phi and psi, they determine how the entire chain moves while the chi, the omega just determines whether we have a cis or trans bond around the peptide bond that's there. And then the chi ones are gonna determine how the side chain rotates. In this case, it's a pretty boring one because it's just gonna be a C3 group rotating but if you have a large group here, that might tell us something about that particular group's packing and degrees of freedom, but that effect is gonna be local. If you have to make your bet, it's that it thinks it's always gonna be the phi and the psi that are most important because they influence chains along the entire chain and in contrast to omega, we do know that they tend to take on different values. So now we're done, right? Now it's just a matter of throwing this in a solution of water and then find things. Well, not so fast.