Now that we know some of the basics about monosaccharides, I want to focus on a new property, which is their chirality, also known as their absolute configuration. It turns out that some parts of monosaccharide chirality are actually much easier than the chirality of other molecules. But then, other elements of their chirality are really confusing and honestly a little bit messed up. So what I'm going to do in this section is I'm going to clear up both sides of chirality for you, okay? So let's go ahead and get started. Monosaccharides can come in one of two forms: either in the dextral rotary form, also known as the D form, or the level rotary form, L form. And if you remember, these terms are borrowed from the chirality section of your textbook where we learned about optical activity, and we learned that dextrotary molecules rotate light clockwise and level rotary molecules rotate light counterclockwise in a polarimeter. Remember that another property about optically active molecules is that opposite rotations are always enantiomers of each other. So, the positive and negative rotations were always going to be perfect enantiomers of each other, and that's a parallel that holds true for this section. The D enantiomer or the D sugar is always going to be an enantiomer of the L sugar. So this is true. Unfortunately, this is where the parallels from optical activity end for sugars because it turns out that when sugars were discovered, it was based on a lot of guesswork by Emil Fischer back in the 1800s. He didn't have a polarimeter; he didn't have all the monosaccharides. A lot of this was simply guesswork. So it turns out that he made these rules of D and L before he actually knew that there were a lot of exceptions to that rule. So it turns out that D doesn't specifically mean positive rotation anymore, L doesn't mean negative rotation. So you might be asking, "Well Johnny, then what does it mean?" Well, I'm going to explain that to you, but just so you know, there is no correlation anymore between D being a positive rotation and L being a negative rotation. You should just think of them as two categories of sugars. Okay? Now, the way you determine D or the way you determine L on a monosaccharide is decided by the penultimate carbon. Such a fancy word! What does that mean? Penultimate is just a word that means "second to last," okay? So the penultimate carbon is simply going to be the last chiral carbon of your molecule, and that's going to determine if it's a D or if it's an L. Now note to self, this penultimate carbon is essential. It's the C5 carbon usually, but it depends on the length of the monosaccharide. In a six-carbon sugar, it's going to be the C5, but it's very important; it's going to be used as what we call a stereodescriptor later in this chapter. So these two words are going to be synonyms of each other. The penultimate carbon and the stereo descriptor carbon are synonyms of each other. So just keep that in mind, okay? I'm going to use both of those words interchangeably. So how do we know if it's a D or an L? Well thankfully, this is one of the really easy parts of monosaccharide chirality. All it is is this, the D configuration is going to be any sugar that has the penultimate or stereo descriptor OH pointed to the right. Okay. So let's look at glucose here. Glucose, which OH is the stereo descriptor? It's going to be the last chiral carbon, so it's going to be this one right here. That is the last chiral carbon. It's the one that's furthest down. Again, it happens to be C5 here. We're going to learn how to number carbons soon or number sugars soon, but in this case, this would be 1, 2, 3, 4, 5. Okay. So that's my penultimate carbon or my stereo descriptor, and what we know is that if it faces to the right, that's going to be a D-glucose. So I'm going to just put that right there, this is D-glucose. And then if it faces to the left, that's going to be an L-glucose. And that makes it so easy guys because you can just remember that L is left. Okay. D is right, L is left, you have no excuse to mess that up because the L's it's a double L. Okay? Now guys, it turns out that usually that means that it's going to be the D is going to be an R configuration at that last sugar or at that last chiral center and usually L is going to be an S at that position. Okay. But there are exceptions to that as well. So just so you know, this works for most sugars but for other molecules, biomolecules like amino acids, it's not going to work anymore. So for right now, you can use that rule, but the rule that I really want you to remember is just right and left. That always holds true, okay? So let me just kind of prove to you that the R and S works here, but then just don't worry about it for other types of molecules. So for example, here's this chiral center and what I would see is that let me just erase these numbers so that we can pick our priorities. So our priorities would be 1, the carbon I mean the OH is number 1, then this must be number 2, this must be number 3, and this must be priority number 4. So that means that my rotation goes from 1 to 2. From 2 to 3, I ignore 4. It looks like it's an S but since my 4 is on the horizontal that means it's going to be an R. Okay? So remember guys what I'm doing is I'm using the rules for determining chirality and Fischer projections to determine that's an R. So what I'm doing here is I'm showing you guys that yes, the D happens to be an R but it's not always going to work for other types of biomolecules, okay? The one thing that you should always remember is that to the right is D and to the left is L, and that's always going to hold true. Cool? Alright guys, so we're done with this concept let's move on to a practice problem.
28. Carbohydrates
Monosaccharides - D and L Isomerism
28. Carbohydrates
Monosaccharides - D and L Isomerism - Online Tutor, Practice Problems & Exam Prep
All monosaccharides come in dextrorotary (D) and levorotary (L) forms. You may have heard these terms before when learning about optical activity. Another way to descrbise these D and L forms are enantiomers of each other.
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Monosaccharides - D and L Isomerism
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Problem
ProblemProvide the generic name, including stereochemistry, for the following monosaccharide:
A
L-aldotetrose
B
D-aldotetrose
C
L-aldopentose
D
D-ketopentose
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PRACTICE PROBLEMS AND ACTIVITIES (5)
- (c) Draw d-idose, the C3 epimer of D-talose. Now compare your answers with Figure 23-3.(d) Draw the C4 “epimer...
- Indicate whether each of the following structures is d-glyceraldehyde or l-glyceraldehyde, assuming that the h...
- Categorize the following monosaccharides as d or l.(a) <IMAGE>
- (••) Starting with a Fischer projection of d-glucose (and other sugars), switch only the stereocenter that gav...
- (••) Is the compound d or l?