 So, I already told you about the aromatic side chains, but I want to introduce this concept properly. Aromatic side chains, such as phenylalanine tyrosine, but in particular tryptophan, is going to have this strong effect that the mere size of the aromatic rings means that they have to stand up in the bilayer. Tryptophan is even cooler than the other ones, because tryptophan has this one large hydrophobic part, the six-member ring here, that will have to stick and then we also have this polar part here with the NH group. The dashed line you see here is the NH group making a hydrogen bond. So, that hydrogen bond is going to have to go up to an oxygen either in another lipid or in water, but that hydrogen bond means that we literally can't pull it down, because if I pull it down I would break the hydrogen bond and that's very costly. I also can't pull this up, because I try to pull this up I would take that entire aromatic ring and expose that to water or something, that's not going to happen. So, tryptophan in particular, it's an anchor, it's a double anchor, it's an anchor upwards so that I can't pull it down and it's an anchor downwards that I can't pull it up. So, just to illustrate this concept, this is called anchoring and it occurs particularly for tryptophan and that's why we had the W in the WOLP helices. It's an awesome way to ensure that a part of my helix actually occurs in the head group.