Now when it comes to charged amino acids, we're going to say that they represent polar amino acids, and they contain groups that are electrically charged at physiological pH, meaning they possess a positive or negative charge. Now here if we have an acidic group, we're gonna say it contains a negative carboxylate anion group. And if you have a basic group, then you're gonna be a positive amino group. Our memory tool here is dragons eat knights riding horses. So if we take a look at this image, remember dragons tend to have a negative connotation attached to them. So the dragons, which are negative, are connected to our carboxylate groups. Knights, on the other hand, are positive because they defend us against the dragons. They're positive. They represent the basic groups.
O- The first letters here represent the one-letter code for each of these amino acids. Now, here if we take a look, for our acidic ones, it's a giveaway when we're talking about their acid forms; aspartic acid, glutamic acid, acids in the name, we know that they're acidic amino acids. So here, as carboxylic acid, they'd be O. But, if we're talking about them being in physiological pH, the pH will be high enough that they're gonna lose their H+. So they actually will exist as O- . In these forms, their names are now aspartate and glutamate. Here we're gonna say the one-letter code, for both of them are the same it would be ASP, and then here would be D, D for dragon. Here, glutamic acid now becomes glutamate. We're gonna say here this is GLU, and this is E, dragons eat.
Lysine here is LY, arginine, and histidine are our basic ones. We're gonna say lysine here is LYS. It is different from all the other amino acids because it doesn't use the first letter of its name, and it is not phonetic in origin. It uses K as its one-letter code. So here, knights. And so here, it has to be positive. For it to be positive, the nitrogen has to make 4 bonds, so it has to be an NH₃ group here and positive. Arginine or arginine, depending on how you want to pronounce it. We're gonna say ARG. It is phonetic in origin, so R is its one-letter code. Again, remember, nitrogen needs to make 4 bonds in order to be positive. So this nitrogen, which is already making 2, would have to more hydrogens. NH ₃ + Histidine, his, it is unique. It's one-letter code is H. Nitrogen, here we see it making 3 bonds already, so it only has one H on it to make it positive. Now going back to the acidic ones, we're going to say that they exist as the conjugate forms of their acidic and basic forms. So aspartic acid, physiological pH, pH is too high, can exist as an O- group. Same thing with glutamic acid. Physiological pH, the pH is actually too low for the basic ones. So instead of these nitrogens being neutral, they're gonna have a positive charge. So that's why we have to make sure they're each making 4 bonds by adding enough hydrogens so they gain a positive charge on the nitrogen. So remember, these would represent our charged polar amino acids, which can be further broken down into acidic and basic amino acids.