Which form of aspartic acid in Problem 18.54 is the zwitterion...

Question

Which form of aspartic acid in Problem 18.54 is the zwitterion? What is the pI for the zwitterion?

a.

b.

c.

Accepted Answer

Welcome everybody. Our next question says which of the following forms of glutamic acid is the Zwitterion and its isoelectric point P I. So we have four different answer choices with four different forms of glutamic acid and each with API value. So we'll look at each of them and think through our answer here in choice. A, we see that glutamic acid has two carboxylate groups, the one on the backbone and the one that's part of the R group, I'm going to highlight the R group in blue so that we keep straight. What is what here? And we see that the R group for glutamic acid is CH two, CH two and then coo or coo H depending on what form it's in. So the form we have here in choice. A both of the carboxylate groups are carboxylate ion. They have negative charges on the oxygen and our aming group on amino acid backbone is NH two. So actually started to circle it, but I won't circle it because it doesn't have a charge. It has, it's neutral since it only has two hydrogens, then it gives api value of 9.7. Well, even in the name glutamic acid that can help us remember that it's an acidic R group. And if we didn't remember that, just looking at the structure of the R group, we have, it's a carboxylic acid. So this is an acidic R group. And therefore, if we have an acidic R group, we'd expect our P to be low. Recall that the P I is the PH value at which the molecule is electrically neutral. In this case, the acidity of the R group means that you have an extra negative charge from the R group when it's in its acidic form. So it won't be electrically neutral unless that our group is proton, which since it's acidic won't be occurring unless you are at a very low Ph. So P I of 9.7 would be a valued expect of a basic R group. So even without looking any much further at the molecule, we know that choice A must be incorrect. And indeed, we're looking for the Zwitterion. Zwitterion is an I dipolar ion where you have one positive, 11 negative charge canceling each other out. But in this case, this form has a net charge of negative two. So chose definitely not our answer. So let's look at choice B and Choice B, we see that both carboxylic acid groups are in the carboxylate form with negative one charges on the oxygen and the amine group is in the form NH three plus. So it has a positive charge on the nitrogen. It has two negative charges and one positive charge. So it has a net charge of negative one. So it is not this witter I am. So we will eliminate choice B. In addition, the P I here is 4.3. Well, that may be right, that's an A that would indicate an acidic R group. So we won't rule that out definitely. But as we said, the fact that it has a net charge of negative one has ruled out the possibility of choice B being our answer. So now we look at choice C. In choice C, the R group is in the carboxylic acid form still has a hydrogen on the oxygen. So it's a neutral, that neutral over here. But the backbone of the amino acid has a negative charge on the oxygen and a positive charge on the amine. So in this case, are molecules net neutral, the positive and negative charges balance each other out. The R group is ne has no charge on it at this point. And we have api value of 3.2 which would indicate an acidic R group. So shall I see must be our answer since this is our waiter, I am and we can confirm that P I by looking up at a table in a textbook and that will show you that that is indeed the P I of glutamic acid. Finally, to be through, we'll look at choice D in cho D, the two carboxylic acid groups are in their acidic form, they have hydrogens on the oxygen. So they have no charge. The only charge is on the amine, which is in the form NH three plus. So we have a net charge of positive one. So this is not this wood or iron and it says P I of 2.2. Well, that's an acidic P I. So it could be a possibility although we know from looking up that it is not, but that net charge of positive one would definitely rule out choice D. So once again, which of these forms is this W ion and has the correct isoelectric point. And that is choice C which was api of 3.2 and a net neutral charge due to having just a positive charge on the amine and a negative charge on the backbone carboxylic acid but not on the R group. See you in the next video.

Which form of aspartic acid in Problem 18.54 is the zwitterion? What is the pI for the zwitterion?
a.
b.
c.

Key Concepts

Zwitterion

Isoelectric Point (pI)

Aspartic Acid Structure

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