At neutral pH, which of the following amino acids has a net positive charge, which has a net negative charge, and which is neutral? (Hint: Draw the various charged forms of each amino acid before deciding.)
a. Asparagine
b. Lysine
c. Proline

Question

At neutral pH, which of the following amino acids has a net positive charge, which has a net negative charge, and which is neutral? (Hint: Draw the various charged forms of each amino acid before deciding.)
a. Asparagine
b. Lysine
c. Proline

a. Asparagine

b. Lysine

c. Proline

Accepted Answer

Welcome everybody. Here's our next problem identify whether the following amino acids will have a net positive charge, net negative charge or neutral at neutral ph ph H equals 5 to 8. And the amino acids are number one aspartic acid P I equals 3.0. Number two, alanine P I equals 6.0. And number three, glutamine pi equals 5.7. So let's recall what the P I information tells us and how that can help us answer this question, which is P I is the PM which a molecule has no net electrical charge. Keep in mind, acidic or basic molecules can exist in two different forms. So it's at this ph the form it's in has no net electrical charge, the positive charges and the negative charges cancel each other out. So our question asking us in that 5 to 8 ph range, what is the net charge on our amino acid? So with that in mind, we would say we would have a shortcut using these PIS, which is that if the P I is less than five, our molecule have a net negative charge. Whereas if the P I is greater than eight. So outside of this range, our molecule will have a net positive charge. So for the, if we remember this and can think of it this way we can use the P I values to give us our answer quite easily. So for number one Aspartic acid, our P I is 3.0 this is less than five. So aspartic acid will be negative at a neutral Ph number two alanine RPIS six. So since it's within our range, that we're looking for, it will be neutral because that's the whole point, the P I is the ph point at which the molecule is neutral. And then number three, glutamine, the P I is 5.7 and therefore, it will be electrically neutral than we would expect it to be. Now, to explain why this is the case. Let's look at the structure of these molecules. So for aspartic acid, first of all, let's look at what, remember what the backbone structure of an amino acid is. So I'm going to draw it as a skeletal structure and I'm going to draw it with the R group sort of on the side. So there's our, our group and it will be bonded to the alpha carbon. So we'll label this guy alpha and that alpha carbon will have a bond too an NH two group. And it also has that bond to that carboxylic acid group. So another carbon double bonded to an oxygen with an oh Now, this is in the electrically neutral form, but it won't really exist that way. Um in general. So let's think about this. And the kind of easiest way to think through the steps of changing ph of the amino acid is to imagine at first, at a low enough ph that our amino acid is fully proton. Now, obviously, this will depend specifically on the R branch. So in the case of number one aspartic acid, the R branch is ac two coo H. So it is a carboxylic acid group which explains that acidic P I that value that it has. So let's draw our line structure for number one aspartic acid, we have our and we're going fully protein it. So we will have an ho of the carboxylic acid or C double bond O then RCH two and that will be bonded to the alpha carbon. Putting a little dot there to remind me that's the alpha carbon which has a fully proton amine group, which would be NH three with a positive charge on the nitrogen. Again, since that's the basic group, it can accept a proton and then that alpha carbon will be attached to the other carboxylic acid group, fully proton. So C double Bondo and then, oh so this is a fully proton form of aspartic acid. Now let's take it up and we see this has a net positive charge. So now let's take it up to a P of 3.0 which is our P I point. We know this will be neutral. And so we're going to, we're raising the ph, we're going to lose one of these protons. Well, we have two carboxylic acid groups, which one will we lose? Well, the carboxylic acid group attached to the alpha carbon. So part of our backbone is the more acidic because it's so close to that positive charge on the nitrogen closer than our, our um our group carboxylic acid group. Since that has the added carbon, we're going to highlight our group just so we can keep straight. What part of the molecule it is. I highlighted that in blue. So we're going to drop hydrogen from the backbone carboxylic acid. So my, our group still has, it's hydrogen on the oxygen are nitrogen, still has that positive charge. But now the carboxylic acid group has an O minus, it has lost its proton. So we have one positive charge and one negative charge. So as we expected, ph 3.0 the P I point, this is neutral. But now we're going up to a ph of 65 to 8, excuse me, 5 to 8. So we've gone up higher. So now we expect to lose another proton. And in this case, the carboxylic acid group on the R group is obviously more acidic than RNH three plus. So we will lose the proton on our R group. So we'll have an O minus bonded to that carbon, carbon, our next carbon, the nitrogen is still positively charged. And then our second, they're now carboxylate groups, not carboxylic acids. So now we have two negative charges on our oxygens and one positive. So this is a net negative charge at that neutral ph. So you can see how starting from the fully proton form can help you think through what's going on. So we'll do it one more time for alanine just to see how this works with a neutral amino acid because we know that uh number three similar. So we have number two here and we need to recall that an alanine or our group is just a methyl group. So we have our ch three bonded to our alpha carbon which has that NH two and then the carbonel group. So let's draw the fully proton version. So our carboxylic acid already has a hydrogen. We just need to change our NH two group two NH three plus. And this would be the fully proton version because our, our group that methyl cannot have a proton added to it. So the fully proton version has a net positive charge. So that's not the, the state at the P I obviously. So at a ph of 6.0 which is our P I point to the point at which it's neutral. We know we lose one hydrogen. And obviously, in this case, it has to be the backbone carboxylic acid group. So at that point, you have an O minus and an NH three plus. So we see again, the charges balance out. But the reason it's at a much higher ph is because the R group doesn't have a proton to lose the neutral form is when you just have the backbone atoms, the nitrogen and the oxygen in those oppositely charged states. So in that 5 to 8 range, we would expect the alanine to be neutral. And then when we scroll up, going to go back up to our glutamine. So we said that the alanine has and our group of CH three, when we look at glutamine, it has an R group that's a little more complex. It's CH two CH two C double bond to an onh two. So it's an amid an amid can be very, very weakly acidic. Um As you can see by the fact that the P I is 5.7 a little bit lower than alanine, but it doesn't generally act that way. It won't be basic like an a uh an amine because the lone pair is delocalized over that C with, along with that co double bond. So it is much more stable. It's not going to want to ex its loan pair is not there of just looking for a proton. So glutamine having this neutral R group will have a similar stepwise process as the neutral alanine. So once again, when we look at these, we see Aspartic acid is the net negative charge in this neutral situation. So let's look at our answer. Choices. Choice A correctly says aspartic acid will have a net negative. But choice B says aspartic acid will have a net positive. So we can eliminate. Choice B choice C says Aspartic acid will have a net negative charge. So that's correct. But choice D says it will have a positive charge. So we eliminate choice D now let's move on to alanine. Choice A says that alanine will have a net positive charge but we know alanine is neutral. So eliminate choice. A all we've got left is choice C. So let's double check the rest of it. It says number two, alanine will be neutral. So that's correct. And number three, glutamine will be neutral. So that's all correct. And of our three amino acids, what will their charge be in that neutral? Ph range from 5 to 8. Choice C. Aspartic acid will have a net negative charge. Number two, alanine will be neutral and number three, glutamine will be neutral. See you in the next video.

At neutral pH, which of the following amino acids has a net positive charge, which has a net negative charge, and which is neutral? (Hint: Draw the various charged forms of each amino acid before deciding.)
a. Asparagine
b. Lysine
c. Proline

Verified video answer for a similar problem:

Key Concepts

Amino Acid Structure

Ionization of Amino Acids

Net Charge of Amino Acids

At neutral pH, which of the following amino acids has a net positive charge, which has a net negative charge, and which is neutral? (Hint: Draw the various charged forms of each amino acid before deciding.)a. Asparagineb. Lysinec. Proline

Verified video answer for a similar problem:

Key Concepts

Amino Acid Structure

Ionization of Amino Acids

Net Charge of Amino Acids

At neutral pH, which of the following amino acids has a net positive charge, which has a net negative charge, and which is neutral? (Hint: Draw the various charged forms of each amino acid before deciding.)
a. Asparagine
b. Lysine
c. Proline