 So looking at practical structure, it's going to turn out that many of them have lots of alpha helices and others have lots of beta sheets. This one actually has a mixture of both, but to make our lucks easier, we're going to start by looking, particularly these building blocks. The helices and the sheets understanding when they are stable, why they form and roughly how fast they form, before we then, in a few lectures from now, carry on to looking at entire proteins and how we assemble them. We've already looked at helices and sheets, although I then didn't tell you how important they were, but if we, for a second, look at an individual alpha helix, that is stable, these individual straight lines, we occasionally call them a beta strand, but a single beta strand is not really stable. We're going to need at least two of them to form a so-called sheet that we can start to talk about something that's at least semi-stable. In last lecture, we looked a little bit about the hydrogen bonds, and we're going to come back to that, that it's very local interactions here, and you're going to need to trust me when I say that this means that it's going to form fast. It is a very rigid structure when this is formed. There are, if I put two helices like this next to each other, they're going to be very few hydrogen bonds between them, because they're all paired up inside the helix. And that in particular means that if I have one helix here and one helix here, they're going to have to be packed with Lenard-Jones interactions or so, so there are, it's locally constrained, but it's not really going to create constraints to some other part very far away in the protein. Beta sheet, it's kind of obvious, as our opposite. It has, these hydrogen bonds are close in space, but they're not close in sequence, so it's non-local interactions. It's much more floppy, flexible. You can bend the sheets this way without really breaking anything. And all hydrogen bonds are between strands rather than within a single strand. And that means that in particular if these two strands are far away from each other in sequence, we're now going to create global constraints that ties up the structure and reduces entropy a lot. This will be more complicated to understand how it folds and everything, so I would suggest that we start with the alpha helix and make sense of that, but not just hand-waving, but studying the free energy of alpha helix formation.