Isoleucine has the zwitterion structure shown. Draw the structure and give the net charge of isoleucine that will predominate at the indicated pH values (pI = 6.0).

b. pH 6.0

Question

Isoleucine has the zwitterion structure shown. Draw the structure and give the net charge of isoleucine that will predominate at the indicated pH values (pI = 6.0).

b. pH 6.0

Accepted Answer

Alright, hello everyone. So this question says given the Zwitterion structure of phenylalanine, which of the following correctly shows the net charge of phenylalanine at a PH of nine. Now, the p of phenylalanine is equal to 5.75. And here's the structure of Aladdin on the screen. So first and foremost, recall that P I is an abbreviation for the isoelectric point and the isoelectric point is the PH at which a particular species exists as a sweater ion. Now, a sweater ion is a species that does have parts of it with positive and negative charges, but the net charge, the charge of the compound as a whole is zero, it's neutral overall. So it makes sense to say then that as the Ph of the solution is deviating from the isoelectric point that balance between protonation and dep prote nation is going to change, right, the ionization state is going to change. So let's talk about that. In this particular example, we have a ph of nine, which means that the PH is actually greater than the isoelectric point of 5.75. This means therefore that the environment is basic, right? It's a basic environment, which means that generally speaking dero nation is going to be favored. Now, as a more general rule, we can say that when the ph is above the isoelectric point, then species are going to generally carry a negative charge. So let's go ahead and apply this to phenylalanine directly looking at the Twitter ion itself. We can see that on the left side, we have a positively charged amino group. And on the right side, we have a negatively charged carboxylate group. So in basic conditions where protonation is favored, the carboxylate group already has a negative charge, which means that it cannot be further deproteinate. On the other hand, we have an amino group that currently has a positive charge and can definitely lose a proton. So thats precisely what's going to happen at a PH of nine, the amino group is going to lose a proton and for every proton loss, the charge decreases by one. So the charge of the amino group is going to decrease from positive 1 to 0. So here when redrawing the structure of phenylalanine at this new Ph, I would draw nitrogen with only two hydrogens and not three, but my alpha carbon and the car box. So they group are going to stay the same and the side chain is going to stay the same as well. And just for the sake of completeness, right, it's worth mentioning that if the PH were to be acidic were to be below the isoelectric point, then protonation would be favored instead, and the carboxylate group would get proton to produce a neutral carboxylic acid just drawing out my benzene ring. So now let's talk about net charge earlier in the form of the Zwitterion. The reason why the charge ended up being zero overall is because the positive charge of the amino group canceled out the negative charge of the carboxylate group. Here in the new structure reflecting phenylalanine at ph nine, the only group of phenylalanine that has a charge is the carboxylate group with a negative one charge, which means that the net charge is going to be negative one. So now we have our correct answer. And so with that being said, thank you so very much for watching and I hope you found this helpful.

Isoleucine has the zwitterion structure shown. Draw the structure and give the net charge of isoleucine that will predominate at the indicated pH values (pI = 6.0).

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b. pH 6.0

Verified Solution

Key Concepts

Zwitterion Structure

Isoelectric Point (pI)

pH and Charge Relationship