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Carbohydrates - Epimers, common names

Created by Ryan Scott Patton.

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Video transcript

- [Voiceover] We've spent a lot of time on the front end of carbohydrates talking about their stereochemistry, especially of that last chiral center. And again, it's because it plays a large role in the biological function of these molecules. For example, we humans are enzymatically programmed to break down and digest the D sugars. For that reason I want to spend at least one last short amount of time trying to clarify the questions that I originally had when learning about carbohydrate stereochemistry and nomenclature. First I need to clarify that D and L refer to stereochemistry but they don't speak to the overall optical activity of the molecule. As an example let's take a look at D-threose. D-threose has an aldehyde functional group and it has four carbons, so it's an aldotetrose. But you can see that the last chiral center down here has its functional group, this hydroxyl group on the right side, so it's a D carbohydrate, D threose, but it turns out that there are actually two chiral centers here and whenever we have n chiral centers, whatever number of chiral centers we have, then we have two to the n possible stereoisomers. And in this case there are two chiral centers so we have four possible stereoisomers. And it turns out that this particular stereoisomer actually has an overall optical activity such that it rotates plane-like counterclockwise, as opposed to clockwise like you would see with most R configurations. So even though this is D it's actually a negative, it gets a negative sign for its optical activity. So this is D(-)threose, and again it's D because this lowest chiral center here has an R stereochemistry, so it's a D carbohydrate. The second big thing I want to clarify is that it's important to note that the D and L configurations of a particular carbohydrate are enantiomers, which means they differ at every chiral carbon, not just the last one. We can take a look at this in the case of glucose. Glucose, again, is an aldehyde carbohydrate so it's an aldose and it's got six carbons, so it's an aldohexose. And this is the D configuration. The L configuration is gonna look like this. You can see again it has six carbons. Nothing's changing there. But as we reflect it across this mirror every single chiral carbon is going to be the mirror image. So this is L glucose. And again the big thing that I want to clarify here is that it's not just this last chiral center down here. It's not just this last chiral carbon that is flipped for the D and L. The D and L glucose are true enantiomers. Enantiomers, which means that they're complete mirror images; They differ at every single chiral carbon. Now that being said if the D-aldohexoses, these glucose, if the D- and L- aldohexoses are enantionmers, that means that all of the D-aldohexoses have to be diastereomers of each other, because they're not superimposable and they're not mirror images. I know that's confusing but I've drawn out here all of the D aldohexoses and we'll take a look at what I'm talking about. We have the D aldohexoses here and there's eight of them that I've drawn. In the case of glucose, up above, I'm gonna flip back up to it for a second. You see that D-glucose and L-glucose are enantiomers, they differ at every single carbon. Now, all of these are stereoisomers but they differ at maybe just one. They don't differ at every single carbon from glucose. Here's glucose down here. You can see D-allose, it's just different at this one chiral carbon right here. Or you can see D-galactose up here. The only difference is this C4 chiral carbon from glucose. What you see is that these aren't mirror images and they're not superimposable, so all of the D-aldohexoses are diastereomers. It's the same thing for all of the L-aldohexoses, they're all diastereomers of each other. And you can carry that through the ketopentoses, all the D-ketopentoses would be diastereomers of each other and they would have a partner in the L-ketopentoses that would be their enantiomer. So again, this is a terribly confusing idea but I really think the best way would be if you could just pause the video for a second and take a look at all eight of these and notice where they're different, and notice that they're not different at every single carbon so they can't be enantiomers. I've said enantiomers and diastereomers too many times already, I'm sure. I mentioned just a minute ago that glucose and galactose are different only at the C4 carbon. Remember we've got one, two, three, four, five, six. And similarly with glucose, one, two, three, four, five, six. So the only carbon that these differ at is the C4, and because they just differ at one carbon we have a special word for these, and they're called epimers. Epimers are diastereomers that differ at one chiral center. That's a vocab word that's probably going to come up several more times as you look at carbohydrate chemistry. You can this thought of D verse L carbohydrates to the next level with critical thinking if you consider all of the stereoisomers for an aldohexose. So again we're talking about aldohexoses right now. How many chiral centers do these aldohexoses have? We can count. There are one ... So one, two, three, four. That's not numbering that, I'm just counting the chiral centers because this carbon up here, the carbonyl carbon, is double bonded to an oxygen so it's not a chiral center. And down here this carbon is bound to two different hydrogens so it's not a chiral center. So all of these aldohexoses have four chiral centers. That means they have two to the four or 16 stereoisomers. Half of those are gonna have to have this OH at the bottom on the right side and the other half would be left. So half of 16 is eight and that's how we get to this idea that there are eight D-aldohexoses. That's a thought that you can use and you can translate that into pentoses. Pentoses are gonna have three chiral centers so there's gonna be ultimately eight, and there would be four D and four L. The last thing I wanna do is cover the common names for the five most commonly seen monosaccharides. I've similarly pre-drawn their structures in and I'll give you their names and the mnemonics that I was taught to remember them by. The first one that we have right here, number one, is ribose. The way I remember this, this is a pentose, it's an aldopentose, it's got an aldehyde and five carbons. It's an aldopentose and all of the substituents, all of these hydroxyl groups are on the right side. I remember that ribose is "all right." The next one we have, hopefully you can see here because we've drawn it a couple different times is glucose. I should mention that this is D-glucose, again, and I should mention that this is D-ribose here. The way that I remember glucose is actually a little bit racy, so keep in mind that I do not support flipping people off with your middle finger, but if you look at this, man, it sure does resemble somebody flipping off people, so you can say, I don't know, whatever insult you want to glucose. I'll just write some expletive marks here to glucose. You can remember that glucose, we'll just pretend that we're really frustrated with it and we're cursing it out. And again I don't condone you using your middle finger, but thank goodness that organic chemistry can redeem even the most heinous of societal insults. We can remember that D-glucose looks like if we're holding-- Kind of, this is our pointer finger, and you can curl your finger up and stick your middle finger out with the fingernail down towards the page and I'm sure you can make the connection of how your fingers resemble glucose. That's my mnemonic for that. This next one is mannose, and again it's D-mannose. And if you position your fingers in the same way that you were with glucose and now you just extend your pointer finger as well. Now we've got two fingers extended and then two curled up. We can see that it's like a man holding his gun. We're the man and we're holding our gun and that's D-mannose. So man with a gun. And again this is an aldohexose just like glucose and to keep using that vocabulary these are diastereomers of each other. This next one on the list is galactose. It's kind of lame but the way I remember this is that D-galactose is the C4 epimer of glucose. So galactose, I've got this C4 epimer of glucose. Down here this is the only carbon, the only chiral center where it differs from glucose so I remember it's the C4 epimer. And then last but not least we have fructose, and this made an appearance in an earlier video. We've got D-fructose and the way I remember D-fructose is that it's the ketose of glucose. So the ketose of glucose, and you can see that it very much resembles glucose except that instead of an aldehyde it has a ketone functional group. These are maybe the most common monosaccharides that you'll see in an organic chemistry and biochemistry context.