 This itself leads to an interesting pattern. You might think that positive and negative charges are opposite sides of the same coin, which is kind of true, but they behave very different in a membrane. Lycine and Arginine for that matter, they're positively tarred side chains. They will have this pattern that I showed you that they're sticking out a bit. We're going to call that snorkeling in a few slides. That particular means that they can interact with water, but they don't have to stretch as far as the water. They can interact with those negatively charged oxygens in the carbonyl groups, which is the charged group located closest to the interior in the lipid bilayer. But note that oxygen is negative. It can pair up with a positively charged head group, but it cannot pair up with a negatively charged amino acid, such as aspartic acid. Aspartic or glutamic acid, they are negatively charged side chains instead, which again would seem just like the positive charged one, just with different sign. This helix, or these side chains here, they cannot interact with the carbonyl groups. They have to stretch that all the way out to the water or the phosphates. Do you see what the difference is? This is a far larger distortion of the entire helix and the membrane around it. This is going to be much, much, much costlier. So it's harder to introduce negatively charged residues on the inside of membranes than positively charged ones. Now, friend of order would say, can't we just forget about that by not trying to introduce any charged ones at all? But it's going to be functionally relevant to introduce them at some points. And I'll show you why in 20 minutes or so. Just remember, when we need a charge in the membrane, it's better to have positive charges that we work with than negative charges. And finally, just to complete this, here we have a residue that was not charged, but tyrosine, kind of similar to tryptophan that it has an aromatic ring. Do you see how these rings locate? This is not a simplified model or my illustration. This is a snapshot from a computer simulation. The rings orient in the way so that they intercalate between the lipid chains. They would never be in this orientation because that would require them to push away five or six different lipid chains. So this is a very particular pattern for aromatic side chains, positively charged side chains and negatively charged side chains. Let's minimize that a bit and take this into the lipid environment here. The reason for the positive charges is that they interact with these groups. The negatively charged side chains, on the other hand, had to stretch all the way up here or even the water. And the tyrosines in particular, they need to orient this way to avoid interfering too much with the chains.