Two of the 20 common amino acids have two chiral carbon atoms in their structures. Identify these amino acids and their chiral carbon atoms.

Question

Accepted Answer

Hi, everyone. Here's our next question. Which of the following amino acids has two chiral centers identify the two chiral centers using an asterisk. And we have five different amino acids shown here. So we need to recall what a chiral center is, which would be a carbon atom that is bonded to four different groups in a tetrahedral arrangement. So you need two things, four different groups around your carbon hat and tetrahedral geometry. So with that in mind, let's start looking through our amino acids. We'll start with number one, we have our amino acid backbone with our NH three plus the alpha carbon, the carbon carbon and then O minus. So I'd like to start by highlighting the carbon atoms in our molecule that we should be looking at. So we obviously have two carbon atoms in the amino acid backbone, the alpha carbon and the carboneum carbon. And in our particular R group, we have a chain with CH two sh. So there's that carbon atom in our R group. So let's look through these. Well, the carbon carbon is immediately out of the running because it has a double bond to oxygen. So it is not tetrahedral. It's a in a trigonal planar arrangement. Note that this carbon carbon is present in every amino acid. So we won't even bother highlighting it in our other molecules because we just know it's never going to be a chiral center. Now, let's look at our alpha carbon. Well, as part of the amino acid backbone, it has the nitrogen from the Nhu plus group, it has a hydrogen and it has the carbon carbon. So it already has three different groups bonded to it. In every amino acid, you just have to look at the R group. So the alpha carbon is almost always going to be a chiral center since you've started with three different things. Unless your R group happens to be the same as one of these, then you will have a chiral center at that alpha carbon. So we'll put an asterisk there since it has four different groups and there are four single bonds. So it is a tetrahedral arrangement. Finally, we'll look at our, our group carbon, but we know without even looking at everything, it has two hydrogen atoms. So it does not have four different groups bonded to it. So it is not a chiral center. So number one is not our answer because it has only one chiral center. So let's move on to number two. So let's highlight our carbons. Once again, I'm I'm going to skip the carbon carbon since I know from looking at number one, it won't be a cro center. So I'll highlight my alpha carbon and then I look at my R group, it has a three carbon chain. So there's three carbons and then it has a methyl branch on the first carbon. So there's 1/4 carbon. Well, right away, I can rule out any of the carbons that have more than one hydrogen and three of those carbons in the R group do. So we have at the end of the chain ch two CH three. So those are out. And then of course, our methyl branch has ach three as well. So that's three carbons ruled out. Now, let's look at our alpha carbon and it's pretty easy to see that it is indeed chiral because that our group is this chain of three with a methyl branch, obviously not identical to any of the things in the amino acid backbone. So we'll put an asterisk by that alpha carbon. And then now let's just look at the first carbon in the R group that's attached to the alpha carbon. And we see we'll list out it's bonded to a hydrogen to our alpha carbon to a methyl group. And then in terms of the rest of the chain and AY group, so we see those are four different things. So this first carbon in our, our group is also a chiral center. So the area we go in number two, we have two chiral centers. So I will circle that as our answer, but it just says which of the following. So perhaps there's more than one. Although since it's labeled number one, we'll see if there's, since it's labeled one through five, we'll see, perhaps there's only one. Let's look on to number three. Again, ignore our carbon carbon and highlight. We have the alpha carbon. And then in our R group, we have a CH two and then we have a benzene ring. So this is a six carbon ring. But when we look at those carbons, each of them participates in a double bond. So none of them in the ring are tetrahedral. So none in ring tetrahedral. So none of the carbons in the ring will be a Cairo center. So let's start with the easy one. Our alpha carbon, obviously, the R group is not identical to any parts of the amino acid backbone. So this is a carbo center. And now we just need to look at that first carbon in the R group, it has the alpha carbon, but it then has two hydrogens attached to it. So it is not a chiral center since those two hydrogens are identical. So we cross out number three, it just has one chiral center. Let's move on to molecule. Number four, we have our alpha carbon. And then in our, our group, we have something very similar to. Number two, we have a three carbon chain with a methyl branch. But the methyl branch is on the second carbon of the chain rather than the first. So once again, we have four carbons in the R group, but we can rule out one to three of them for having more than one hydrogen atom. So there's ach two and then two terminal ch three s. So let's look at our alpha carbon next. And that chain is indeed not identical to any parts of the backbone. So, asterisk on the alpha carbon, now, let's look at the only carbon in the R group that doesn't have more than one hydrogen, this middle, the second carbon in the chain and look at what it's attached to, it has one hydrogen, then it has the CH two plus, I'll put a acid. So CH two plus the amino acid backbone, but then it has two methyl groups. So it does not have four different groups. So it is not a chiral center. So we'll put underline that two methyl groups. So number four also has just one chiral center. Despite its similarity to number two, let's look at number five. So highlight our carbons, ignoring our carbon carbon. We have the alpha carbon and then our, our group is an alcohol group. It has cho H and then the ch has a methyl group attached to it. So we have two carbons in the R group and let's take a look at them. Well, right away, we notice we have a terminal method group. So that has three hydrogens and is not a chiral center. Let's look at our alpha carbon. Nope, that our group is not identical to anything. So it's a chiral center. And now let's look at that carbon. That's the first, the one attached to the alpha carbon in our R group, it has a hydrogen atom, it has the alpha carbon, it has a methyl group and it has an alcohol group. So this is also a chiral center. So we do indeed have two molecules here that have two chiral centers. So number five is also correct. No, I just realized I marked number four, I circled it when I should have crossed it out and read that number four has only one chiral center. So number five has two chiral centers and number two has two chiral centers. Look for those two asterisks in each of those molecules. So we have two of these molecules that are amino acids with two chiral centers. See you in the next video.

Two of the 20 common amino acids have two chiral carbon atoms in their structures. Identify these amino acids and their chiral carbon atoms.

Verified video answer for a similar problem:

Key Concepts

Chirality

Amino Acids

Chiral Carbon Atoms in Amino Acids