So now that we've talked about the groupings for all 20 amino acids and each of their individual R groups, we're now going to talk about zwitterions. So it turns out that the common backbone for all 20 amino acids is predominantly a zwitterion at physiological pH. And recall that physiological pH is right around a pH of about 7. And recall also that when we say predominant structure, what we really mean is that this structure is in the highest concentration amongst all the other types of structure. And so this fact here is actually gonna become a common known fact, that's gonna be part of our basic knowledge of amino acids, even though it might be new information to you right now. Now it turns out that a zwitterion, so the definition of a zwitterion, is just a dipolar molecule that bears or that has 2 groups that are oppositely charged, so 2 groups of opposite charge. And the charges on those two groups are a result of acid base reactions or H plus transfers or proton transfers. And so in our example, notice that we have the common, structure for all 20 amino acids. So we have our alpha carbon in the center here, we have our central hydrogen, we have our carboxylate group on the right, and our amino group on the left, and then down below, we have our r group. And so, what you'll also notice is that we have the 2 groups bearing charges highlighted. So on the left here, we have our amino group, which notice has a positive charge on it, and then on the right, we have our carboxylate anion which has a negative charge on this oxygen shown there. And so because there are 2 groups that have opposite charged on the same molecule, this is a zwitterion. And the backbone of all 20 amino acids is a zwitterion, specifically at physiological pH, which is physiological pH, which is right around pH of 7. And so this is again gonna be common knowledge that is gonna be very basic knowledge that we're gonna be referring to throughout our course. So it's good to get this introduced. And so we'll be able to get a little bit of practice utilizing this concept in our next practice video, so I'll see you guys there.
2
Problem
Problem
Which shows the proper structure of Leu at physiological pH?
A
B
C
D
3
concept
Zwitterion
Video duration:
8m
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Video transcript
So in our last lesson video, we said that the predominant structure of the amino acid backbone is a Zwitter ion specifically at physiological pH which is a pH of 7. But the question is, what is the predominant structure when the pH is not at physiological pH? Like if the pH is pH 1 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 pKa is a quantitative measure of the strength of an acidic hydrogen. And the greater the pka 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 pka value to the pH of the solution. And so when we independently compare each pka value to the pH of the solution, we're 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 pKa of the acid in order to determine the predominant species, and whether that species is protonated 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 2 new pieces of information that I want you guys to know. The first is that alpha amino groups, or amino groups that are part of the backbone of an amino acid, have pKa values in the ballpark range of about 9 to about 10.5. Now, the second new piece of information that I want you guys to know in this video is that alpha carboxyl groups or carboxyl groups that are part of the backbone of an amino acid have pKa values that are in the ballpark range of about 2. And so it's really important to know these ballpark pKas 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 gonna 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 7, which is physiological pH, and the predominant structure of the backbone is a zwitterion where we have a protonated amino group over here on the left with a positive charge and a deprotonated carboxyl group carboxylate group on the right with a negative charge. And so this positive charge cancels out with the negative charge and the backbone bone net charge, specifically at pH 7, is going to be 0. Now if we take this pH of 7 and we drop it down to an acidic pH of pH 1, then we're going to need to use our previous knowledge to be able to determine the predominant structure, and we're gonna have to independently compare each pKa to the pH of the solution. Now notice that we're given a pKa for the amino group of 9, and a pKa of the carboxyl group of 2. 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 1, so the pH of 1, is less than the amino group p k a. So it's less than the amino group p k a, which is again equal to 9. And so when the pH of the solution is less than the p k a, that means that the concentration of conjugate base is gonna be less than the concentration of conjugate acid and the conjugate acid will be predominating. And so the majority of the molecules are going to be protonated and have an additional hydrogen. And so we can expect our amino group to be in the protonated form, which is the NH 3 plus form, and we'll have a positive charge. So now, if we do the same thing, but for the carboxyl group, notice that the pH of 1 is still going to be less than the carboxyl group pKa. And again, the carboxyl group pKa is given to us as 2. And so, again, when the pH is less than the pKa, the concentration of conjugate acid will be greater and the majority of the molecules will be protonated. So we can expect our carboxyl group to be protonated. And the protonated carboxyl group is just gonna have an Oh 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 1. And so that means that the backbone net charge is just going to be plus one at a pH of 1. So now, if we go ahead and increase the pH all the way up to a pH of 12, then we can go ahead and perform the same strategy. And so notice that when the pH of the solution is equal to 12, that is actually going to be greater than the amino group p k a. So it's greater than the amino p k a, which is again given to us as 9. And so when the pH of the solution is greater than the p k a, 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 deprotonated and have one less hydrogen. And so we can expect our amino group to be in the deprotonated form, which is gonna be the NH two form, which has no charge on it. So now we can do the same with our carboxyl group, and so when the pH of the solution is equal to 12, that's still going to be greater than the carboxyl group pKa, which is again given to us as 2. And so again, when the pH is greater than the pKa, the concentration of conjugate base will be greater and the majority of the molecules will be deprotonated, so we expect our carboxyl group to be deprotonated. And the deprotonated carboxyl group is gonna be a carboxylate anion which has a negative charge on it. And so notice that when the pH is equal to 12, the predominant structure of our backbone only has one charge on it, so that means that the backbone net 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 pH is equal to 7, right around physiological pH, that corresponds to our, x axis here, the pH of 7, right at this position. And you can see that, most of our molecules are going to be in the zwitterion form because the concentration is at its highest here. And so, the zwitterion form, has a neutral net charge of 0, like we indicated down below. Now if we go ahead and drop the pH down to an acidic pH of 1, 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 protonated. And so, you can see that when both groups are protonated in our backbone, that the backbone has a plus one charge, and that corresponds, 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 deprotonated state in this blue that corresponds with this blue line here. And so, both groups will be deprotonated, and when both groups are deprotonated in the backbone, our backbone has a net charge of negative one. So up above we can indicate negative one. And so this here concludes our refresher on our, lesson here, and we'll be able to utilize these two new pieces of information problems moving forward. So I'll see you guys in those videos.
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example
Zwitterion Example 2
Video duration:
2m
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Video transcript
Alright. So here's an example problem that says, atph2, which is the predominant structure of valine? And we're given these 2 pKas, pka 1 which is 9.62 and pka 2 which is 2.32. And so we know from our previous lesson videos that valine is a nonpolar amino acid with a v shaped r group. And so looking at all of these r groups down below in our answer options, which are highlighted in blue, notice that they all have the same structure and the same v shape. So they all have the correct R group for valine and we're going to need to determine the correct answer by determining the predominant structure of valine's backbone at a pH of 2. And so recall from our previous lesson videos that alpha amino groups have pKas in the ballpark range of about 9 to 10.5. And so pka 1 here must be the pKa of the amino group. And also recall that alpha carboxyl groups have pKas in the ballpark range of about 2, and so pka 2 here must be for the carboxyl group. And we know that we can determine the predominant structure by independently comparing these 2 pKas to the pH pH of the solution. And so we can start with the amino group p k a. And so we know that a pH of 2 is less than the amino group pKa, which is equal to 9.62. And so we know from our previous lesson videos that when the pH is less than the pKa, that the conjugate acid form predominates and the conjugate acid is a protonated form. And the protonated form of the amino group is going to be an NH 3+. And so we can eliminate any answer option that does not have an NH 3 plus for the amino group. And so this one, notice has an NH 2 plus, so we can cross that off. And this one just has an NH 2, so we can cross that off as well. And notice that both option a and d have the correct NH 3+ form, and so we're gonna need to look at the carboxyl group next. And so, we know that a pH of 2 is also less than the carboxylpkaof2.32. And so again, when the pH is less than the pKa, the conjugate acid form predominates, and the conjugate acid form is protonated, so we expect our carboxyl group to be protonated. And so notice that the carboxyl group for option a is not protonated, so we can go ahead and cross that off. And looking at option D, notice that it is protonated, and so that means that D here is going to be the correct answer. And that concludes this practice or this example problem. So, we'll be able to get some practice in our next couple of videos and I'll see you guys there.
5
Problem
Problem
Fill in the groups for the predominant structure of Ala at pH 13?
Video duration:
2m
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6
Problem
Problem
Fill in the appropriate groups for Asp at pH 4.3.
Video duration:
4m
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7
Problem
Problem
Draw the predominant structure of Arg at pH 6.5? (pK a1 = 9.04, pKa2 = 2.17, pKa3 = 12.48).
Video duration:
6m
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8
Problem
Problem
At what pH would an amino acid bear both a neutral -COOH and a -NH 2 group?