 So we're almost done looking at how different amino acids behave in a helix, but I want to take one more step. This far we only looked at the head group region, and that's partly because the hydrophobic part is boring, right? We just want hydrophobic residues there. But maybe. Maybe I can try to introduce something maybe not charged, but at least polar in the center of the membrane. Let's pick serine. It has a small OH group. This serine side chain, do you see what it's doing, the red one? Well, I did put it in the middle of the membrane and it's not going to be happy about that. But what this side chain effectively does, this is in a computer simulation, but do you see how the OH group there is effectively sharing the hydrogen bond to the backbone in the helix? So while it's not going to be happy, it's not going to be an unpaired hydrogen bond sticking out in the membrane. If you know your amino acids, there is another residue very similar to serine called threonine, also has an OH group, and we see the same effect with threonine. So for both these residues, they're not charged, so that while they are polar, it is possible to assert them in membrane. I will be paying a bit, but in many cases I can compensate for that if I make the rest of the helix really hydrophilic, lots of leucines on the other side chains. That will balance things so that in total it will still be better to put it in the membrane. And then once they are in the membrane, they will have this pattern where, sure, the individual side chain is not happy, but it can get by, because it's at least sharing a hydrogen bond. There is a reason why I'm showing you that, but I will have to keep you on hold for like 30 seconds there while I'm going to introduce one more concept before you understand the beauty of it.