Isoelectric Point

In this video, we're going to discuss a really important property of amino acids called the ISO Electric point. So the ice electric point is the pH at which an amino acid has zero net charge. It's also defined as the pH, at which an amino acid has the greatest concentration of sweeter ions present, which is what I have here. Now, those two definitions actually mean the same exact thing, because think about it. What type of net charge does his with her eye on half? Zero, Remember, by definitions, Witter ions have zero net charge, right? So if I have the maximum concentration of Twitter ions, what is my net charge is gonna be zero. Okay, so it's two different ways of saying the same thing, which is that we're looking. We're kind of tuning it. We're gonna tune that pH to figure out What's the pH that I can set it at for that amino acid so that I have the most amount of Twitter ions present a k. So I have the least amount of chart of net charge is present. Okay, so they're all zero. So for a generic amino acid, the P I would just be calculated by averaging. That's how you spell it, averaging the two function of the PKK of the two functional groups. Now what do I mean by generic? Well, generic would just be an amino acid that you don't know what the side chain is. It's basically, and amino acid with side chain are. It's unknown. I don't know what it is, so let's go ahead and do this exercise first. Before we talk about specific amino acids, let's try to approximate what the general p I would be for just a generic amino acid. So the way that this would work is that we would just have to use approximate P k A values. So what did I tell you guys? Is the approximate P K value for the O negative on an amino acid? It's around two right, so let's go ahead and fill that in here. It's around two in the pink area now for the blue area, the basic area. What's the approximate P K for an amino amino acid? For actually, it's called the ammonium groups as a positive charge. It's around nine. Remember that? So for just some random amino acid where you don't know what the R group is. The ice electric point would be taken by just averaging the PKK's of both of these groups. Now, this number would come out to 11 over to which, if you type it into your calculator, that's going to give you a nice electric point of 5.5. Okay? And it turns out that that actually is close to many of the answers. For lots of the amino acids are ice electric point will be around 5.5. That means that if you set your PH toe 5.5 and put these amino acids in, almost them are gonna be as witter ions because it's gonna be in between the two ph is of your p k one or in between the PKK one and P k two. It's going to get that midpoint so that most of them are twitter ions. Okay, now it turns out that some of them are not gonna be 5.5. But this is where most of them are gonna be around. Okay, so how do we calculate the exact is electric points? We're going to discuss that in the next video

so all 20 amino acids fall into three different buckets of ways to calculate the ice ice Electric point. I just wanna look at what those categories are. First, the first category is non acidic or basic amino acids. That's group one. Group two is acidic and basic amino acids, and Group three is just Sistine. Sistine is its own exception, so it's gonna be in its own bucket. So what that means is that if we were to work backwards from this, the ice electric point of Sistine Sistine is in its own group, the ones that are in the acidic and basic amino acids group. What are they? Well, we should have that memorized. We should know that the acidic ones R E d. And we should know that the basic ones are K h R. Right. So that means that if my amino acid is in that bucket, I use this technique. If it's in this bucket, I used this technique. So that means that what types of amino acids are in the first bucket, almost all of them. So I'm just gonna put here everything or I'm just gonna put here all minus e the okay age are and see. Okay, so all of them, all 14 of them, besides those will be in this first bucket. So let's talk about that first category. So if you have one of the immuno acids that's not in these special categories, you could just calculate it as a generic amino acid. Which is the best part. All you do is you just look up the PK a values and average that means 14 of are really straightforward. So federal Allan E. Is that one in this first group. Yes, because it's not acidic. It's not basic. It's not assisting. So what I do is I have to look up the exact values for federal Valentine. Let's go ahead and do that. Now. What I see is that my values are, um, 1.83 and 9.13 So let's go ahead and write those down. So it was 1.93 and nope, 8.1 point 83 and nine 0.13 I almost got that wrong. Okay, Dyslexia. You did not win. So, um so basically, those are my two values. So here we're just gonna have to use a calculator in average them. What that means is that I'm going to add 1.83 plus 9.1 3/2. And whatever that's equal to will be the answer. So I'm just gonna calculate it right now on my phone. So 1.83 plus 9. equals 10.96 divided by two equals that the Ice Electric a point of federal Allen is Cool, awesome. So let's go ahead and do the next one.

in this next category, we have acidic and basic side chains, and these are gonna have to use a different rule. And the reason is because you can't just average them. There are now three ionized able groups, and when you have three, you don't just average three of them. You still are only supposed to use to. So how do you pick the two that you're supposed to use? Well, what you do is you pick the two similar groups. You look for the two most similar groups and then those of the PKK's that you use. Now, I already drew out the three p. K. S for you that I got from the sheet. But just be aware that we're only gonna use two of these. One of these is gonna get scratched out. It's not gonna get used for ice Electric point. So the place that you always start is with the side chain because the side chain is always gonna be one of your two. So let's circle aside, chain. Highlight it for sure. I'm gonna use this peak A. That PK is six. So for sure, that's gonna be one of them. But now I have to decide. Is the nitrogen group more similar? Or is the oxygen group more similar to the history and side chain? And the answer is the nitrogen. Okay, Why guys just has to do with Adams. It's that easy. If you have nitrogen in your side chain, you pick the ammonium to be the most similar. If you have oxygen's in your side chain, you pick the oxygen to be most similar. So if this had been glue tannic acid that has carb oxalic acids on both sides, you pick the O's. But if this was Argentine, which has nitrogen on one side, then you pick the nitrogen and the nitrogen is being similar. Okay, In this case, history in has what? Nitrogen? So I would pick the nitrogen group. Okay. What that means is that I'm going to use this as my second group, so I'm gonna use this. PK is my second p a. And I'm scratching out PK one because I'm not gonna use it. That's not the most similar. And now we average these out. So this part's easy 6.0 plus 9.17 divided by two. So we're just gonna use your calculator again. Six plus 9.17 equals 15. 17. Divided by two equals at the is electric point of histamine is 7.59 I'm rounding up. Okay, Awesome. So we're done with this one. Let's move on to the next category.

so technically Sistine doesn't need to have its own category. But I put it here because I would. I realized that could be the most confusing one. And the reason is because Sistine is the on Li Ion Izabal side chain that has neither an oxygen or nitrogen, right? It's the only one that has a hetero atom that's different. So you could be really confused trying to pick which one similar. It's the same rule, but you might just have a hard time applying it, you might say. Well, is the sulfur more like the nitrogen or is it more like the oxygen? I just wanted to make it clear to you guys that for the purpose of this exercise, it is more similar to the oxygen. And it makes sense because, remember, the oxygen is either gonna have ah, hydrogen or a negative charge, right? Same thing with the sulfur. It's either gonna have a hydrogen or a negative charge is never gonna have to H is in a positive. Okay, so this is just basically an extra little point to say that you average the sulfur and the oxygen as the ones being similar. It's basically the same thing as the acid in based rule is just a different application of it. Okay, So that means that the PKK's that I'm using here. Well, I'm gonna use these two groups, which means the PKs. I'm using our PK one and PK three. And I'm not using p k two. Okay, so now we're just gonna add them together. So it's gonna be 196 plus 818 over to. So I'm gonna do that in my calculator. So I'm getting 10. 14. Divided by two equals a isil electric point of 5.0 seven. Cool. Awesome guys. So we're done with this page. Let's move on to the next.