So you think you're getting the hang of this. This is kind of making sense. And now I'm going to throw a wrench into the whole thing. Because it turns out that there's an extra complication which is that 7 of the 20 amino acids that you learned actually also have ionizable side chains. What that means is that when you are planning out the zwitterionic form, if it has positives, negatives, etcetera, you need to also look at the side chain and you have to figure out if the side chain is going to be protonated or deprotonated, etcetera. On top of that, some professors, some universities make you memorize these 7 pKa's. So I'm going to give them to you and I don't really have the best memory tools for them. It's going to be up to you to figure out if you need to memorize these or if they're going to be given to you. Most of the time, 90% of the time, they're given to you just like the others, so you don't have to worry. But if you happen to have a professor that wants you to memorize them, then you're going to need to figure out a way to memorize them. Okay? So let's go ahead and move to the next page. I'm going to move to basically back to the amino acid breakdown page. Okay? So this is back to the amino acid breakdown and now we're going to be filling in these boxes over here and the 7 ionizable pKa's or side chains are the ones that have white boxes. So we're going to be filling in numbers here. Okay? Cool.
Another disclaimer. It turns out that not only does not everyone have to memorize them, some people do, some people don't, but there are also differing values depending on your professor, depending on what website you look at. Some will say 8.3 and some will say 8.4. It's like really stupid. So I personally don't think it's worth memorizing unless your professor makes you because no one really fully agrees. I'm going to go ahead and give you the values that I found were the most popular ones. But if your professor or textbook requires you to memorize a slightly different one, use that one. Okay? So cysteine, 8.18. I'm not going to give you ways to memorize this. I'm just going to write them down and you figure that out if you need to. Tyrosine, 10.07. Aspartic acid, 3.65. Glutamic acid, 4.25. By the way, that kind of makes sense. Right? That aspartic would be more acidic than glutamic because it's closer to the nitrogen, so it's going to have a stronger inductive effect and this one's further away. Kinda makes sense. Lysine, 10.53. Histidine is exactly 6. That one seems to be pretty universally agreed on. It's almost always 6. And then arginine, 12.48. Okay? So just I'm not going to help you memorize these. You probably don't even need to, but just have them written down so we can use them in the following problems. We're going to referring to this sheet a lot because we're going to need to get all our pKa values from here. Okay? Now, I happen to know that the next example is with Lysine. So let's get these Let's write down these numbers now so that we can use them on the next page. And I'm actually going to write them down myself so that I can use them on the next page. So lysine, the carboxylic acid is 2.18. The amine is 8.95. As you can see, again, they're close to 2.9, but they're a little bit off. And then the lysine side chain is 10.53. So now let's go over to the example, figure out what the predominant form of lysine looks like and what's the net charge at a pH of 8.5. Cool? So actually, let's just do this together. Okay? Actually, okay. No. I'll let you guys try it first, and then in the next video, I will solve it for you.