 So that was a bit of a long derivation. Let me recap the take-home message for you. Why don't we have very short secondary structural elements in proteins? Because if you look at free energy, virtually all of them look like that. Your first paying-to-go uphill, the Helix initiation barrier, right? Then you start going downhill, but it's not until you get to a certain length that the net delta G value is going to be smaller than a zero. It's only when you get to this length, it's going to be advantageous to form an alpha helix, minimum length. Why are alpha helix is not infinitely long? Well, if I gave you sequence that for some reason would be an alpha helix, then that would be stable as an alpha helix. It is just that the possibility of that occurring in nature is not zero, but very very small. Try to pick 1,000 residues and never ever, by mistake, picking the half of the residues that prefer to not be a helix. Well, that's like tossing a coin 1,000 times and never getting tails. Not impossible, but exceptionally unlikely.