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Carbohydrates - absolute configuration, epimers, common names

Video transcript

- Okay, so we spend 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. So, 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. So, 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. So, as an example, let's take a look at D-threose, and 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, you know, 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 kind of 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 kind of a negative sign for its optical activity. So, this is D minus threose. And again, it's D because this lowest chiral center here has an R stereochemistry. So, it's a D-carbohydrate. Now the second big thing that I want to clarify is that it's important to note that the D and L configurations of a particular carbohydrate are enantiomers, which mean they differ at every chiral carbon, not just the last one. So, we can take a look at this in the case of glucose. So, 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 going to look like this. So, you can see again it has six carbons, so nothing's changing there, but as we reflected 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. So, 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 enantiomers, 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. And I know that's confusing, but I've drawn out here all of the D-aldohexoses, and we'll just kind of take a look at what I'm talking about. So, we have the D-aldohexoses here, and there's eight of them that I've drawn So, in the case of glucose up above, I'm going to flip back up to it for a second, you see the 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. So, here's glucose down here, and you can see D-aldose. Well, 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. And so, 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 thought 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. Okay, I've said enantiomers and diastereomers too many times already, I'm sure. Now 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 this C4, and because they just differ at one carbon, we have a special word for these, and they're called epimers. So, epimers are diastereomers that differ at one chiral center. And that's just kind of a vocab word that's probably going to come up several more times as you look at carbohydrate chemistry. Now you can take kind of this thought of diverse L-carbohydrates to the next level with critical thinking, if you consider all of the stereoisomers for an aldohexose. Again, we're talking about aldohexoses right now, and how many chiral centers do these aldohexoses have? Well, you can count. There are one-- So, one, two, three, four. Now, that's not numbering it. 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, and that means they have two to the four, or 16 stereoisomers. Now, half of those are going to 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. And that's just kind of a thought that you can use, and you can translate that into pentoses. Pentoses are going to have three chiral centers. So, there's going to be ultimately eight, and there would be four D and four L. So, the last thing I want to do is cover just the common names for maybe the five most commonly seen monosaccharides, and I've similarly kind of pre-drawn their structures in, and I'll give you their names and kind of the mnemonics that I was taught to remember them by. So, the first one that we have right here, number one, is ribose, so ribose, and the way I remember this, this is a pentose. It's an aldopentose. It's got an aldehyde and five carbons. So, it's an aldopentose, and all of the substituents, all of these hydroxyl groups are on the right side. So, I remember that ribose is all right. Now the next one we have, hopefully you can see here, because we've drawn it a couple different times, is glucose, and I should mention that this is D-glucose again, and I should mention that this is D-ribose here. But 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 insults you want to glucose. I'll just kind of like write some kind of expletive marks here to glucose. And you can remember that glucose, we'll just pretend that we're really frustrated with it, and we're kind of 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. And so, we can remember that D-glucose looks like, if we're holding, if kind of this is our pointer finger, and you can curl your finger up and kind of 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. And so, that's kind of 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. So, now we've got kind of two fingers extended and then two kind of curled up. We can see that it's like a man holding his gun. So, 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. And then this next one on the list is galactose, and 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 the C4 epimer of glucose. So, 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 kind of made an appearance in an earlier video. And so, 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, and these are maybe the most common monosaccharides that you'll see in an organic chemistry in kind of biochemistry context.