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.
