 find the hydrogen bonds and the donor and accept the nomenclature, we can look a little bit at hydrogen bonds and real molecules. I already mentioned water, right? And in that movie that I showed you, I mentioned that water, a perfect ice crystal at zero Kelvin would have exactly two full hydrogen bonds per water. That is not as simple as you might think, because each hydrogen is participating in one hydrogen bond. But the oxygen also has two electron pairs. So each water molecule is participating in four hydrogen bonds. But it's donor for two of them, and it's acceptor for two of them. So it's kind of, it's participating forming four half hydrogen bonds. So the total number of hydrogen bonds is going to be two per water under the ideal scenario. Now, once we start cranking up the temperature and going to, say, room temperature, so when we have liquid water, what's going to happen is that the number of hydrogen bonds is going to go just slightly above two, but as long as we're ice, it's not going to increase a whole lot. And then when we move over to the liquid phase, there will be a jump because the atoms will now, the molecules will now start to diffuse and move relative to each other. But it turns out that almost all the hydrogen bonds are still intact. Liquid water has an average of 1.7 hydrogen bonds. So if you do H2O, aqueous phase is 1.7 H bonds. And if instead they do, I'll say X-ray for a crystal, that would be roughly two H bonds. Not quite. If I were to boil the water then, if I would go all the way to gas phase, I would have zero hydrogen bonds. That's not quite true, but there will be some of them formed transiently, but in principle in gas phase, things are so far away from each other that they do not interact. We already talked about DNA. Remember that I mentioned that inside the DNA, the recent DNA paired up the way they did was because we had this basis. Purins and purimidines, A, G, C and T. And these four specific hydrogen bond patterns from a purine to a purimidine and either two hydrogen bonds or three hydrogen bonds under the normal Watson-Crick based pairing. That's what gives DNA its specificity. If it was, if the hydrogen bond was very weak and easier to break, the DNA would not maintain its structural integrity. And there are a ton of hydrogen bonds in this DNA spiral. And that's of course why it's stable and it doesn't deteriorate, which we should be fairly happy about. Because if it deteriorated quickly, we would form tumors and have errors in our genetic code. But it's not just DNA. It's going to turn out that almost all the molecules we work with in this class have hydrogen bonds. We're going to talk about protein structure. On the very far there, you have a so-called alpha helix. And in the alpha helix, we have tons of hydrogen bonds formed along a staircase. It's going to involve those peptide bonds I showed you before. The other panel there shows so-called beta sheets. We'll introduce the next week. And the beta sheets also have a ton of hydrogen bonds. So there are two aspects that we can learn already from this lecture. Remember how I said how the torsions are important because the torsions determine the rotational degrees of freedom. That is, they determine how a molecule can move. On the other hand, the strongest interactions that actually form or break are the hydrogen bonds. Note that form or break is an important modifier there because if I literally tore bonds apart, those interactions would be much stronger. But those interactions are so stronger that they are brick walls. And I won't push my head through a brick wall. Sorry, not even for a lecture. But the hydrogen bonds are so strong that they're important to form interactions, but they are weak enough that you can actually break them under some conditions. For instance, if I'm changing temperature or conditions. And that's why this is going to be such a miracle that we're going to look at when do hydrogen bonds form, can that explain when things are stable or not. And the part that allows the molecule to reach those different states will be the torsional degrees of freedom. So again, you need to know what are the important degrees of freedom in most biomolecules and what are the most important interactions to stabilize degrees of freedoms. Hydrogen bonds