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Current time:0:00Total duration:11:15

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 it 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 carbohydrates 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 three o's and D 3 us has an aldehyde functional group and it has four carbons so it's a it's a aldo tetris 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 3 O's but it turns out that there are actually two chiral centers here and when we ever we have you know in and chiral centers whatever number of chiral centers we have then we have two to the N possible stereo isomers and in this case there are 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 counter clockwise as opposed to clockwise like you would see with most our configuration so even though this is D it's actually a negative it gets kind of a negative sign for its optical activity so this is D minus 3 OHS 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 or an anti which mean they differ at every chiral carbon not just the last one so we can 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 aldo hexose and and this is the 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 reflect it across this mirror every single chiral 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 DML the D and L glucose are true in the interiors 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 and we'll just kind of take a look at what I'm talking about so we have the D aldohexoses here and 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 that D glucose and L glucose are enantiomers they differ at every single carbon now all of these are our stereo isomers 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 Alice well it's it's just different at this one chiral carbon right here or you can see d-galactose up here the only difference is this see four 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 and the keto pentoses all the d keto pentoses would be diastereomers of each other and they would have a partner and the L keto pentoses that would be their enantiomer so again either this is 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 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 I've said enantiomers and diastereomers too many times already I'm sure now I mentioned just a minute ago that that glucose and galactose are different only at the c-4 carbon remember we've got one two three four five six and 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 this thought of diverse L carbohydrates to the next level with with critical thinking if you consider all of the stereo isomers 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 1 so 1 2 3 4 that's not numbering that I'm just counting the chiral because this carbon up here the carbonyl carbon is double bonded to an oxygen so it's not a it's not a chiral Center and down here this carbon is bound to two different hydrogen's so it's not a chiral Center so all of these aldohexoses have four chiral centers and that means they have 2 to the 4 or 16 stereo isomers now half of those are going to have to have this o h at the bottom on the right side and the other half would be left so half of 16 is 8 and that's how we get to this idea that there are 8d 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 3 chiral centers so there's going to be ultimately 8 and there would be 4 D and 4 L so the last thing I want to do is cover just the common names for maybe the five most commonly seen monosaccharides that I've similarly kind of pre-drawn their structures in and I'll give you their names and kind of the 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 Al dipinto so it's got an aldehyde and five carbons so it's an alder pentose and all of the substituents all of these hydroxyl groups are on the right side so I remember that ribose is alright now the next one we hear 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 um keep in mind that 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 a flipping off people so you can you can say I don't know whatever insults you want to glucose I'll just kind of like write some kind of exponent marks here to glucose and you can remember that glucose will 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 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 manis 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 Dean manis oh man with again 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 the it's kind of lame but the way I remember that 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 function group and these are maybe the most common monosaccharides that you'll see in an organic chemistry in kind of biochemistry context