Monosaccharides - D and L Isomerism: Study with Video Lessons, Practice Problems & Examples

Monosaccharides exhibit chirality, categorized as D (dextrorotatory) or L (levorotatory) based on the orientation of the penultimate carbon, the last chiral carbon. If the hydroxyl group on this carbon points right, it is a D sugar; if it points left, it is an L sugar. This classification does not correlate with optical rotation due to historical inaccuracies in sugar classification. Understanding this distinction is crucial for grasping the stereochemistry of carbohydrates, as D and L forms are enantiomers, with D typically having an R configuration and L an S configuration at the penultimate carbon.

Monosaccharides - D and L Isomerism

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.

Provide the generic name, including stereochemistry, for the following monosaccharide:

• A. L-aldotetrose
• B. D-aldotetrose
• C. L-aldopentose
• D. D-ketopentose

Here’s what students ask on this topic:

What is the difference between D and L isomers in monosaccharides?

D and L isomers in monosaccharides refer to the orientation of the hydroxyl group on the penultimate carbon, which is the last chiral carbon in the molecule. If the hydroxyl group points to the right, it is a D isomer; if it points to the left, it is an L isomer. This classification does not correlate with optical rotation (dextrorotatory or levorotatory) due to historical inaccuracies. D and L forms are enantiomers, meaning they are mirror images of each other. Typically, D isomers have an R configuration at the penultimate carbon, while L isomers have an S configuration, although there are exceptions.

How do you determine if a monosaccharide is a D or L isomer?

To determine if a monosaccharide is a D or L isomer, you need to look at the penultimate carbon, which is the last chiral carbon in the molecule. If the hydroxyl group on this carbon points to the right in a Fischer projection, the monosaccharide is a D isomer. If the hydroxyl group points to the left, it is an L isomer. This method is straightforward and does not require knowledge of the molecule's optical rotation properties.

Why is the penultimate carbon important in determining D and L isomerism?

The penultimate carbon is crucial in determining D and L isomerism because it is the last chiral carbon in the monosaccharide. The orientation of the hydroxyl group on this carbon (right for D, left for L) defines the isomerism. This carbon is also known as the stereodescriptor carbon, and its configuration helps in categorizing the monosaccharide as either D or L. This classification is essential for understanding the stereochemistry and biological functions of carbohydrates.

Are D and L isomers of monosaccharides always optically active?

D and L isomers of monosaccharides are enantiomers, meaning they are mirror images of each other and typically exhibit optical activity. However, the D and L classification does not directly correlate with the direction of optical rotation (dextrorotatory or levorotatory). Historically, this classification was based on the orientation of the hydroxyl group on the penultimate carbon, not on optical rotation. Therefore, while D and L isomers are usually optically active, their classification does not indicate the direction of their optical rotation.

Can the D and L classification be used for molecules other than monosaccharides?

The D and L classification is primarily used for monosaccharides and amino acids. For monosaccharides, it is based on the orientation of the hydroxyl group on the penultimate carbon. For amino acids, it is based on the orientation of the amino group. However, this classification is not universally applicable to all biomolecules. For other types of molecules, different systems like the R/S nomenclature are used to describe chirality. Therefore, while D and L can be used for some biomolecules, they are not a universal system for all chiral molecules.