 So the other secondary structure we're going to look at is this beta sheet. In this case we have the amino acid chain going down here that it might be spiraling around here and then it goes up here and then it might be spiraling around here and then it goes down here. Slightly more complicated than the alpha helix but here too you have lots of hydrogen bonds between these so-called strands and together these form an extended two-dimensional structure called the beta sheet. In this case it went up and down. You can actually imagine going down and then curling up and forming a helix or something further in here and then we go back and would go down again. So then the two strands would be parallel and based on that you call sheets either anti-parallel or parallel. There is no simple mathematical equation to describe how the hydrogen bonds here are formed. The question is if this is increasing the only question is this decreasing or increasing depending on parallel or anti-parallel. The beautiful part with beta sheets though it has the same properties here that the side chains will not interfere neither with the hydrogen bonds nor with the amino acid backbone itself. In this case with a bit of reasoning you might be able to see that the side chains are either pointing my way or your way and it's alternating that every second amino acid. So here too that's the reason why it's such a beautiful stable regular secondary structure. And in fact due to this regularity Pauling and Cori actually proposed both the alpha helix and the beta sheet would occur in proteins before anybody had crystallized them. That's a pretty impressive feat I think. Showing and explaining that these structures are stable and regular building blocks after having a structure that's obvious. Predicting it a decade before the structures were determined. That's genius.