 So now that we've gone through the concepts of entropy and free energy, we're gonna be able to revisit amino acids but study them on a higher level. Let me start by drawing one for you. We have the C alpha, we have the NH2 group, and we have a COOH group. Then I have a hydrogen that I'm drawing this way to indicate that it's going out front and the R group that I'm drawing that way to indicate that it's going in the back. There are two things that I wanna share with you here that's important. This would be the way that amino acid might look like if we had it as a salt or something but it's not gonna be the way it looks like in solvent. What's gonna happen in solvent is that this hydrogen will be released to water. So we're gonna end up with a charge group here, COOH minus, and the NH2 group, in contrast that will add a hydrogen. So we're gonna end up with a plus charge here. So unless we have a charge on the amino acid side chain, the entire amino acid will still have neutral charge but we have one ion here that's positive and one ion here that's negative. There is a special name for that. It's called the so-called zwitter ion or they have zwitter ionic properties. That's true for all amino acids in solvent at neutral pH. At other pHs it might be different. The other thing worth considering is that this has a tetrahedral shape with the alpha carbon in the middle and then we have four different groups. If you remember your upper secondary schools chemistry, that's gonna be a so-called chiral center meaning that I can draw the mirror image of that molecule. NH3 plus and then COO minus are in the back and H in the front. There is no way to turn the left molecule here into the right one by mirror rotation. If you don't believe that, go ahead and build them with your small custom molecule building site, block and then see if you can take this one and rotate it into that one, it's impossible. The only way I can do that is by swapping two bonds on the central alpha carbon here. Now, knowing free energy and kinetics, you should say that it's only a matter of probability, right? At some point we will be able to cross that barrier. Technically that's true but for amino acids and chemistry and things that have to do with life science, these timescales are so long that it's effectively infinity. They can't take that amino acid and move to that one. In chemistry, you might remember that we occasionally use names as S and R enantiomers to refer to molecules that have the same chemical composition but different chirality. All the physical properties of these molecules are the same, apart from the fact that they tend to rotate polarized lines in opposite directions and that's usually the way we tell them apart. In chemistry, it's quite different though because remember, these are gonna be interacting with tons of other proteins, large molecules such as enzymes breaking down meat into individual amino acids in my stomach and the potentially antibodies interacting with viruses and everything. And if the fundamental molecule here has a property that has a built-in handedness, that's gonna pass on to the entire proteins. So a certain type of molecule here, if you use that to build an entire protein, will not necessarily be compatible with a protein that can bind the other form. And that is quite different from physics and even normal chemistry. The problem with that S and R enantiomers, there are very strict rules how to name them but when it comes to amino acid, if we strictly follow those rules, that would mean that some of the amino acids are S forms and other are R forms while in practice they all look like this. That has to do with naming rules and sulfur, for instance. What we do instead is that we always stick to definition and then we say that these are L amino acids and those are D amino acids. If we do that, it's gonna turn out that every single amino acid we're gonna stumble upon under normal circumstances, at least in nature, are L amino acids. My entire body is built using L amino acids. That again, are fundamentally slightly incompatible with D amino acids. My enzymes would likely not break down as an entire protein built from D amino acids. That's gonna be important later. Remember this handedness, it will come back but now I think it's time to take a look specifically at the side chains and how they create the diversity.