So in our last lesson video, we said that the predominant structure of the amino acid backbone is a sweeter ion, specifically at physiological pH, which is a Ph of seven. But the question is, what is the predominant structure when the pH is not at physiological pH? Like if the PH is Ph. One or Ph. 12 how do you then determine the predominant structure? Well, it turns out you guys already have the knowledge to be able to answer that question. And in this video, I'm mostly just gonna be refreshing your guys memories of older information that we already covered in our previous lesson videos. And so the first piece of information that I want you guys to recall from our previous videos is that the P K is a quantitative measure of the strength of an acidic hydrogen, and the greater the PKK value is, the weaker the acid will actually be. And so the way that we actually determine the predominant amino acid structure is by independently comparing each PK a value to the pH of the solution. And so when we independently compare each PK a value to the pH of the solution, were then able to determine the predominant amino acid structure not just that physiological pH, but at any given pH and so notice down below. We have the same chart from our previous lesson videos. That explains what we mean by comparing the pH of the solution to the PK of the acid in order to determine the predominant species and whether that species is protein ate it or not. So we'll be able to refresh our memories on how to use this chart a little later on in this video. Now, really, in this video, there are only two new pieces of information that I want you guys to know. The first is that Alfa Amino groups or amino groups that are part of the backbone of an amino acid, have PK a values in the ballpark range of about nine to about 10.5. Now. The second new piece of information that I want you guys to know in this video is that Alfa Car boxful groups or car box of groups that are part of the backbone of an amino acid have PK a values that are in the ballpark range of about two. And so it's really important to know these ballpark PK is because your professors expect you guys to know these, and they're going to be really helpful for us as we move forward and solve practice problems and so down below. In our example, notice that we have this graph that's up above here. And this graph is color coordinated to the predominant structures that we see down below. And so what you'll notice is that this green predominant structure here corresponds with the green line up above in our graph. And this blue predominant structure over here corresponds with the blue line. And, of course, the pink predominant structure over here corresponds with the pink line. And so what we're first going to do is fill out these predominant structures down below, and then we'll come back up and look at our graph and see how it relates. And so what you'll notice here is in the center of our image. Here we have the same exact image from our previous lesson video, and that's because we can see that the pH is right at seven, which is physiological. Ph. And the predominant structure of the backbone is a sweeter ion where we have a protein ated amino group over here on the left with the positive charge and a deep protein ated car boxful group car box, late group on the right, with a negative charge. And so this positive charge cancels out with the negative charge, and the backbone net charge, specifically at Ph. Seven is going to be zero. Now, if we take this Ph of seven and we drop it down to an acidic PH of Ph. One, then we're going to need to use our previous knowledge to be able to determine the predominant structure on. We're gonna have to independently compare each peak a to the pH of the solution. Now notice that were given a PK A for the amino group of nine and a PK of the car box, a group of two. So we have to independently compare. So let's start with the amino group here. And so notice that the ph of the solution, which is one so the pH of one is less than the amino group PKK. So it's less than the amino group PKK, which is again equal to nine. And so when the ph of the solution is less than the P. K. That means that the concentration of conjugate base is gonna be less than the concentration of conjugate acid and the conjugal as it will be predominate ing. And so the majority of the molecules are going to be protein ated and have an additional hydrogen. And so we can expect our amino group to be in the protein ated form, which is the NH three Puss form, and we'll have a positive charge. So now if we do the same thing, But for the car Boxer group, notice that the pH of one is still going to be less than the car boxing group K. And again the car box group Pekka is given to us as to. And so again, when the pH is less in the PK, the concentration of can you get acid will be greater and the majority of molecules will be protein ated. So we can expect our car boxer group to be protein aided and the protein ated car Boxer group is just gonna have an o. H at this position, which is gonna be neutral. And so notice that we only have one charge here when our backbone is at a ph of one. And so that means that the backbone that charge is just going to be plus one at a ph of one. So now if we go ahead and increase the Ph all the way up to a Ph of 12 then we could go ahead and perform the same strategy. And so notice that when the pH of the solution is equal to that is actually going to be greater than the amino group p k A. So it's greater than the amino PK, which is again given to us as nine. And so when the pH of the solution is greater than the PK, the concentration of conjugate base will be greater than the concentration of conjugate acid. So that means that the majority of the molecules are going to be de protein ated and have one less hydrogen. And so we can expect our amino group to be in the deep rotated form, which is gonna be the N. H to form, which has no charge on it. So now we could do the same with our car boxer group. And so when the pH of the solution is equal to 12. That's still gonna be greater than the car boxful group PK, which is again given to us as to and so again, when the pH is greater than the PKK, the concentration of conjugate base will be greater and the majority of molecules will be deeper donated. So we expect our car boxer group to be deep protein ated and a deeper donated car Boxer group is gonna be a car Boxley and ion, which has a negative charge on it. And so notice that when the P H is equal to 12 the predominant structure of our backbone only has one charge on it. So that means that the backbone that charge is going to be negative one at a Ph of 12. And so now we can go ahead and relate these predominant structures to our graph up above. And you can see that when the P H is equal to seven right around physiological pH. That corresponds to our, uh, X axis here the ph of seven right at this position. And you can see that, uh, most of our molecules are going to be in this winter ion form because the concentration is at its highest here. And so the sweater ion form has a neutral net charge of zero, like we indicated down below. Now, if we go ahead and drop the PH down to an acidic ph of one, uh, that will correspond right here. And you can see that the majority of our molecules, the highest concentration, is going to be where both groups are protein ated. And so you can see that when both groups are protein ated in our backbone that the backbone has a plus one charge. And that corresponds, uh, to this pink line right here. And then. Of course, if we take our pH and we increase the pH all the way up to a Ph of 12 that's gonna correspond right here on our X axis. And you can see that the majority of our molecules are going to be in the deep rotated state in this blue that corresponds with this blue line here. And so both groups will be deep rotated. And when both groups are deep rotated in the backbone, our backbone has a net charge of negative one so up above, we can indicate negative one. And so this year concludes our refresher on our lesson here, and we'll be able to utilize these two new pieces of information in our practice problems moving forward. So I'll see you guys in those videos.
