L and D Amino Acids - Online Tutor, Practice Problems & Exam Prep

All standard amino acids possess an L configuration, which corresponds to an S configuration in organic chemistry. The S configuration indicates a counterclockwise arrangement around the chiral center. For practical purposes, focus on recognizing configurations visually rather than calculating R/S each time. Key visual cues include placing the amine group down on a wedge for S configuration or left in Fischer projections for L configuration. Understanding these configurations is essential for studying proteins and their functions in biochemistry.

Representations of L-Configuration

In this video, we're going to be discussing the chirality or what's also known as the configuration of the standard amino acids. So let's get started. As previously discussed, all of the standard amino acids that are derived from proteins have the same exact type of chirality center. And that is what's in biology or biochemistry known as an L configuration. But in organic chemistry, we know this better as an S configuration. So just so you know, L and S don't always mean the same thing. But specifically in proteins, they do. So what that means is if you specifically have an amino acid that is in an S configuration or counterclockwise configuration when you calculate it out in organic chemistry, that will be an L. But that may not apply if you try to apply it to a different type of molecule like a sugar. That would be a completely different type of configuration. Now, this does imply if there's another type of configuration because remember, for every S, there's also an R. And the name that we give in biochemistry for the R configuration of an amino acid is a D amino acid. So let's just write that down. A D amino acid, also known as an R configuration, is possible, okay? It's the enantiomer of the original S. However, these are not isolated from proteins, so you don't have to worry about these too much. You may see them come up in a practice problem or it's something theoretical. But for all practical purposes, we are going to always be sticking to this configuration, the S or also known as the L configuration. Awesome.

So the point of this video is not to do a bunch of RNS naming because that takes up a lot of time. What I'd love for you to accomplish in this video is to give you some shortcuts so that instead of having to calculate RNS every time, you can just look at the molecule and just by looking at it, you'll know whether it's an R or it's an S. Okay? So what I've done here is I'm representing 4 different ways that you might see the common L configuration of the standard amino acids, okay? So the first one is if you see the amino acid with the amine sticking down, okay? So basically, the amine is sticking down. You can see that the NH₂ is coming down off of the amino acid. If you see it in that configuration which we're going to be using that type of format of drawing it, we're going to use that kind of rotation often where we have the side chain facing up and the amine facing down. If the amine is facing down, you want to put it on a wedge. If it's on a wedge, that means that this chiral center will be an S chiral center, okay? Now, I'm not going to prove it to you but if you want, you can pause the video and calculate it out and you will see that that indeed is an S. And remember that S equals L, so this would be an L.

Now, what about if the side chain is facing down? So instead of the amine facing down, what about if the side chain is facing down? Well, this is actually the way that I had it represented on the previous page and in the video before this, we were looking at a lot of amino acids that had the amine facing up and the R facing down. Well, if the R is facing down, then the way that you want to represent it is on the dash. That will also mean that this chiral center is in the S configuration. Well, what about if you rotate it? What if you rotate it so that the R group is instead facing up? Well, if the R group is facing up, then you're going to want to put it on the wedge in order to indicate the S chiral center. Okay?

And finally, what about if you have it on a Fischer projection? We're going to have to use some Fischer projections here. Remember that you can any type of practice with Fischer projections, you can look them up on the Clutch Prep search bar and you can do more practice. But this is just a quick tip. If you wanted to draw and you wanted to make sure it's L or S configuration, you put the amine on the left. And an easy way to remember that is that L is left. Now, L actually does not stand for left, but it just works out nicely that when you want to draw a Fischer projection of an amino acid and you want to make sure it's an L configuration, just put the amines to the left and then you're all taken care of. That's all you need to do. Well, amine facing left and obviously follow the other rules of Fischer projections which is that the most oxidized atom or the carboxylic acid goes at the top and then the side chain goes straight down. As long as you do that and have your amine to the left, you're golden.

So I know this might seem like a lot to memorize. You might be thinking, isn't it easier just to do RNS every time? No, it's not because we're going to be working so much with these amino acids. You want to be able to recognize it right by looking at it. You don't want to have to calculate RNS every time. So in these next practice problems, I hope that I'll make it clear to you why it's easier just to try to memorize these than it is to do RNS every time. Okay? So let's go ahead and move on to an example.

Converting Bondline to Fischer Projection

Convert the amino acid histidine into a Fischer projection and then indicate if that chiral center is either R or S. So guys, you're not at the point where you need to memorize what histidine looks like yet. There it is. It's given to us. But what I want to do is draw the Fischer projection without calculating the R and S of the original molecule. We're only going to calculate it after to make sure that we did it right. So I guess first step, let's draw the backbone of our Fischer projection which is going to look something like this. Okay? And now we need to determine, are we going to draw an L or a D amino acid? So I look at this amino acid and how can I easily tell if it's L or D? Well, what I see is that the amine is facing down and the amine happens to be on a wedge. And if the amine is facing down on a wedge, that means this is an S which is an L. So that means that I didn't even have to calculate it to plan it out. I already know that this is an L which means that I can put m

Provide the Fischer Projection representation of D-Aspartic Acid. Indicate if the chiral center is R or S.

Provide the Fischer projection representation of d-aspartic acid and then after you've drawn it, indicate if that chiral center is r or s. So guys, this one was actually pretty easy because it said it in the name. You could actually kinda cheat and just look and say, oh, this is a D-amino acid. So that's the way that I need to draw it on my Fischer projection. But I just want to remind you that if I didn't put it in the name, you could have also known this was D. How? Well, because this is another situation where the amino group is facing down. And notice that instead of being wedged, this time it's dashed. So, when the amino group is facing down and it's dashed, that is d. Now, I don't want you to memorize that even though that sounds so convenient because you don't need to. We don't need to memorize any of the d's. Just memorize the l's and then you can always infer the d from there. So I know that this should be r, which is d, but I was already given that information here. So I'm just reaffirming what you already know. So let's go ahead and draw this out. So here's my Fischer projection backbone. And what do I need to add? What groups do we need to add? I want you to tell me. Come on. Let's do this together. So the top should have what? My carboxylic acid. Very good. And my bottom should have what? My side chain. So let's actually just draw that in now. So my side chain would be CH2 which is this guy right here. And then that's connected to this carboxylic acid. So let's go ahead and draw that like this. Is that fine? Cool. You could have rotated it however you want but that's just the way I'm going to draw it. Cool. So now we just need to figure out if the amine is going to the right or to the left. Since this is D, the amine has to go to the right and that means my H is right there. So now we're done. Let's just go ahead and count out our priorities to make sure that everything works. So this would be 1. Oh my God. I keep making that mistake. I'm so used to carboxylic acid being 1. Nitrogen is 1. Carboxylic acid is 2. The side chain is 3. And hydrogen is always 4. So once well, let's go ahead and count out first. So I go from 1 to 2, 2 to 3, 3 to 1. That looks like an s. But I'm actually going to swap that for r because my once again, my hydrogen is on the side which by the way, it's always going to be on side for amino acids, so then you would swap it to R. So this was an R configuration which confirms that we indeed had the right amino acid to start with and that we drew everything correctly. Great. So let's move on to the next practice problem.

Convert the following Fischer Projection of L-Proline into a bondline structure.

Convert the following Fischer projection of L-proline into a bond line structure. So once again, this is a pretty easy question in the sense that the chirality was already given to you in the name. We're calling it L-proline. So you should already know kind of how to draw that. But in the Fischer projection, just notice that you could have also figured out that it was L because you noticed because you are smart. You're paying attention. You're noticing that the N is facing over here on the left side and we said that left, L stands for left even though it really doesn't but we are just saying that for Fischer projections. So, the trickiest part of this question is just figuring out how to draw a cyclic amino acid. So, we're going to go through that now. So let's go ahead and draw out the backbone of an amino acid. Remember that you have a carboxylic acid and you have your alpha carbon. And for just you know, pretty much what we're going to start doing is we're going to start drawing our amino acids with the N facing down and the R group facing towards the right because that's gonna help us to visualize all the different side chains on the right, okay? So what that means is I need to decide the N is going to be right here. I need to decide if that's going to be wedged or dashed. What do you think? Well, it's L so we know that should be wedged, okay? So this is going to be wedged. And then what I see is that I have this is my alpha carbon here. This is also my chiral center, same thing as this, okay, same carbon. Notice that what I have is a CH2. So let's just draw this CH2. And then what do we have? We have these 2 carbons in the middle that are making a loop between the N and the C. So let's just go ahead and add that in now. This is going to be a carbon-carbon bond like that. It's just going to loop back around the same way as the original Fischer projection. Now I don't want this to look like it's on a dash, so I'm going to redraw that as just a straight line. And now you can see that the chirality is preserved by the wedge structure of that nitrogen, okay? So there's just one last thing to add which is that that nitrogen needs 3 bonds, so let's go ahead and add an H here. Also, I mean, just to match, it needs to match the Fischer Projection, right? So the Fischer Projection had NH and this should also have NH. And now, we're done with this question. It does not say to prove that it's RS. I hope at this point you're already somewhat trusting me on that. So we're done with this question. Let's move on to the next video.

Here’s what students ask on this topic:

What is the difference between L and D amino acids?

L and D amino acids refer to the chirality or configuration of the amino acids. In biochemistry, L-amino acids are the naturally occurring form found in proteins and correspond to the S configuration in organic chemistry. This means the arrangement around the chiral center is counterclockwise. D-amino acids, on the other hand, correspond to the R configuration and are not typically found in proteins. They are the enantiomers of L-amino acids and have a clockwise arrangement around the chiral center.

How can you determine if an amino acid is in the L configuration using a Fischer projection?

To determine if an amino acid is in the L configuration using a Fischer projection, place the carboxylic acid group (the most oxidized atom) at the top. Then, ensure the amine group is on the left side. An easy way to remember this is that 'L' can be associated with 'left.' If the amine group is on the left, the amino acid is in the L configuration. The side chain should be drawn straight down.

Why are L-amino acids more common in nature than D-amino acids?

L-amino acids are more common in nature because they are the building blocks of proteins in all known forms of life. This preference is likely due to evolutionary processes that selected for L-amino acids, leading to the development of enzymes and other biological machinery that specifically recognize and utilize L-amino acids. D-amino acids are rare in nature and are not typically incorporated into proteins.

What is the relationship between L/S and D/R configurations in amino acids?

In amino acids, the L configuration corresponds to the S (sinister) configuration in organic chemistry, which indicates a counterclockwise arrangement around the chiral center. Conversely, the D configuration corresponds to the R (rectus) configuration, indicating a clockwise arrangement. While L and S, as well as D and R, often align in amino acids, this relationship does not necessarily apply to other types of molecules, such as sugars.

How can you quickly identify the configuration of an amino acid without calculating R/S each time?

To quickly identify the configuration of an amino acid, use visual cues. If the amine group is facing down and on a wedge, the chiral center is in the S configuration, which corresponds to L. If the side chain is facing down, place it on a dash to indicate the S configuration. For Fischer projections, ensure the amine group is on the left to confirm the L configuration. These shortcuts help avoid the need to calculate R/S each time.