 Remember when I talked about prions and that said that under some conditions the native state which is the biologically active state is in fact not the lowest free energy minimum as Christian Anfelsen predicted from his dogma. In the cases where there is a second state that is even lower free energy what can happen is that after a long initiation the protein can end up in that state anyway. It has to be long because if this was fast the protein would not work and they would be selected against during natural selection and evolution. So why do they form those structures? Well in the individual protein it's simply that the beta sheet is more stable and that would likely not be so disastrous if it stayed as an individual protein that would unfold. But as I mentioned occasionally this tends to extend into larger plaques some sort of structure where one protein adds to another adds to another adds to another which was not quite obvious I didn't tell you how that happened. Here is roughly how it happens. This green protein is one monomer and the blue one is the second one and they are separate chains so if you look at this in a PDB file the green would be say chain A and the blue be chain B. This is a thioester dihydrase which synthesizes fatty acids it's not important. Each monomer here each unit is stable in itself and it will fold and that's important because as we're going to see later on nature does not like proteins that are too large for whatever reason they take too long to fold. So it's important to have small building blocks so called domains that fold independently with the helix in the background. These are just two copies of exactly the same chain they're just rotated 180 degrees but if each of these is expressed in the cell they will fold but then the green strand here can pair up with the blue strand it's actually the very same strand they just happen to be in different copies of the molecule and extend the beta sheets so they're not connected they're not part of the same chain but effectively all these strands anyway become part of the same beta sheets. The fact there is no requirement for there to be a loop here right? This creates a very large structure where immediately getting rid of this edge effects so both these strands are now in the interior of the sheet where they're more stable we talked about that when we started beta sheet formation. This particular protein only has two it's a common way to create timers but what happens in those plaques is likely and warning the speculation mode turned on now. What if this was a small repeating unit where I could add a third copy here and a fourth and a fifth and a thousand and a millionth? At some point these structures are going to become so large that whenever a new monomere the repeating unit is synthesized and expressed in the cell it's going to be very likely that it will quickly find one of the larger aggregates the so-called oligomers the multimers and attached to that and possibly this could be the reason why the growth is somehow accelerating exponentially. I know this is hand waving it's hand waving because we literally don't know the results yet. There are lots of groups that are studying different type of amyloid like diseases and while we've made a lot of progress it's certainly not the sole field yet so please join us and do research you would do a service to humanity. And that's pretty much all I'm going to tell you about beta sheets. If there is one thing I want you to remember the alpha helix is probably simpler we're going to talk more about that now but the beta sheets get their properties from these non-local hydrogen bonds meaning that they're kind of stable as an individual strand all the hydrogen bonds go between adjacent strands which creates these global constraints but they pretty much I can only pack them parallel or anti-parallel the two strands have to go next to each other I can't pack them like that because then they would not be able to form hydrogen bonds so surprisingly this structure is at least conceptually simpler on the cartoon level than that they might appear a bit deviating from that ideality in practice that is mostly a matter of getting used to looking at structures but that probably means that it's time to head to helices.