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MCAT
Course: MCAT > Unit 9
Lesson 8: Nucleic acids, lipids, and carbohydrates- Nucleic acids, lipids, and carbohydrates questions
- Nucleic acid structure 1
- Antiparallel structure of DNA strands
- Saponification - Base promoted ester hydrolysis
- Lipids - Structure in cell membranes
- Lipids as cofactors and signaling molecules
- Carbohydrates - Naming and classification
- Fischer projections
- Carbohydrates - Epimers, common names
- Carbohydrates - Cyclic structures and anomers
- Carbohydrate - Glycoside formation hydrolysis
- Keto-enol tautomerization (by Sal)
- Disaccharides and polysaccharides
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Carbohydrates - Epimers, common names
Created by Ryan Scott Patton.
Want to join the conversation?
- Would it be correct to say that "D"-Triose can also be called "R"-Triose? Or do we need to use "D" and "L" conventions because these are carbohydrates and/or related to glyceraldehyde?
What about saying that (+)-Triose is the same as d-Triose (i.e. dextrorotary)?(4 votes)- You usually use the D- and L- prefixes for Sugars/Carbohydrates and the "R" and "S" prefixes for all other organic compounds. So, I'm pretty sure that "R"-Triose is correct but it is MORE correct to say "D"-Triose.
No, (+) and (-) indicate the optical activity where as "D" and "L" indicate the orientation of the chiral center. So don't get these two confused, they are two completely different things!(8 votes)
- at, isn't it D-TETROSE, not threose? 1:30
I see four carbons in the molecule including carbon in aldehyde and ch2oh (the 1st one)..hmm..(3 votes)- You might be thinking of Triose ( 3 carbon) . Threose is a 4 carbon monosaccharide.(2 votes)
- D mannose and D glucose are epimers, are they not?(3 votes)
- I like to think of Mannose as "Dos Manos" --Two(2) Hands, in Spanish, because it's an epimer of Glucose at C-2.(3 votes)
- At, Ryan mentions that D-threose has the D config because the highest number chiral centre is R. However, this may not always be the case? 2:00
That is, it will not always work out that D = R and L = S...right? It's purely coincidental on the molecule by molecule basis.
Thanks in advance for any clarification :)(1 vote)- By definition, the D isomers of sugars will always possess the R configuration at their highest-numbered chiral center, whereas L corresponds to an S configuration at that chiral center. Thank you for seeking this clarification! :)(2 votes)
- Wouldn't all but one of the L-stereomers also be diastereomers to all but one of the D-stereomers as well? For example, L-Glucose is an enantiomer to D-glucose but all other L-aldohexoses should be a diastereomer to D-glucose, correct? Also, if epimers differ at only one stereocenter, shouldn't there exist an L-aldohexose (2R,3S,4R,5S) that is an epimer at C-5 to D-glucose?(1 vote)
- This is kind of terrible/inappropriate, but I remember GAlactose as "GAng sign." It's just easier because ribose is the only one that's "all right" and all the hexoses are "vulgar."(1 vote)
- What is the difference between anomers and epimers? What kind of stereoisomers are alpha and beta glucose?(1 vote)
- at, does fructose only have 3 chiral centers? 10:56(1 vote)
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.