 So that takes us back to the helix. This is the one we've studied the alpha helix predicted by Pauling and Corey. If you look at this from the top do you see how neatly packed it is? It looks like it's empty space here but trust me there are going to be a few atoms here so it's perfectly packed on the inside. Along the chain we have these hydrogen bonds from residue i to residue i plus four and it turns out there are exactly 13 atoms if we count all of them per turn. There is a nomenclature for that. To specify helices we typically we occasionally call them NM helices where N is the number of residues between adjacent hydrogen bonds and M the number of residues per sorry atoms per turn. So the alpha helix would be what I would call a 413 helix. Nobody calls that 14 it's always the alpha helix. But telling you that that would seem to indicate that there is more than one helix right? That's true. They're not at all as common that this is probably 98 percent of the helices or 95 at least but you could imagine taking this spiral and twisting it harder. So instead of having four residues between hydrogen bonds what have I only had three? This is a so-called 310 helix. Let's draw three 10. We see that occurring in proteins now and then and the one interesting thing is that you have here we have a spiral of the side chains but in this case this is exactly three residues per turn. We're going to have all the side chains line up on top of each other. It's exactly 120 degrees between them. Here is 100 degrees between residues so 3.6 residues per turn. This is not at all as advantageous. You can almost guess here that things are going to be too packed on the inside here. Things will start to clash and I've had to put quite a lot of strain but there are a few cases where this will be advantageous and I might show you that when we talk about membrane proteins. You could also imagine doing the opposite. Take that helix and unwind it a bit to give it more room. That will lead to this helix which is called a 516 helix or a pi helix. Pi helix is a more common name and this is the opposite. It's a bit too unwound here so we've had to put a bit of strain on it and particularly in the very center of here there aren't really enough atoms. It's almost a bit of vacuum there so that is even less favorable than 3.10. You should know your alpha helix and you should at least know that there is a helix called 3.10 but if you had to take your better helix in a protein it's going to be alpha helix. I've only worked with 3.10 helix throughout my career and I've seen pi helix but never really used them.