 So we're going to spend a lot more time looking at proteins in lecture 5, but I wanted to give you a sneak peek at what we can do with all the math you learned today already now. Let's look at the simplest parts of protein structure, the helix. Alpha helix proposed by Linus Pauling in the 1950s. Very nice, beautiful, regular structure where we have the amino acid backbone curled up in a helix. There are some hydrogen bonds here that might be easier to see this way, that the oxidant here makes a hydrogen bond to another amino acid in the backbone, not the side chain part. We can even calculate that there's a very regular pattern here that residue i makes a hydrogen bond to residue i plus 4 always. And that's interesting because if you think about that, normally that amino acid polypeptide would like to be stretched out and floppy. But when you form this hydrogen bond between residues 0 and 4, that locks in residues 1, 2 and 3, right? And same thing, when I'm forming the hydrogen bond with residues 1 and 5, that effectively locks in residues 2, 3 and 4 so they can't move. The reason why I'm sharing that with you is that this is instructive for you to start thinking about already. The reason why we're forming these structures is of course that it has to be favorable in terms of free energy. But to understand them, you need to reason about enthalpy and entropy. And already now you can do that. What is stabilizing the structure? What is potentially destabilizing it? Will the number of hydrogen bonds formed change when we form it in vacuum versus water? Once they form, what will happen with entropy? Lock, that sounds bad, we're probably losing entropy here. Is there a barrier to forming this? In that case, is it going to be high or low barrier? Well, that is related to the expected time it will take, right? And you should be able to reason about that already now. The other part I would like to point out, do you see all the side chains here? The CH3 groups in this case, they all point out from the helix. And that's one of the reasons why such a beautiful regular structure. The side chains will not interfere with the amino acid backbone.