Monosaccharide Configurations: Study with Video Lessons, Practice Problems & Examples

Understanding monosaccharide configurations involves two primary systems: the Cahn-Ingold-Prelog system for R/S designations and Fisher's convention for D/L configurations. The Cahn-Ingold-Prelog system assigns priorities to chiral carbons based on atomic numbers, while Fisher's convention relates configurations to the orientation of hydroxyl groups in glyceraldehyde. Typically, D configurations correspond to R configurations, and L configurations correspond to S configurations. The overall configuration of a monosaccharide is determined by the highest numbered chiral carbon, emphasizing the importance of these systems in carbohydrate chemistry.

Monosaccharide Configurations

In this video, we're going to begin our discussion on monosaccharide configurations. And so, of course, in your previous organic chemistry courses, you guys learned how to determine the configurations of chiral carbons. And so you may already remember from your previous courses that configurations of monosaccharide chiral carbons can be designated using two different systems. The first system is the Cahn-Ingold-Prelog system, which uses the letter R and the letter S to designate the configuration. And the second system is Fischer's convention, which uses the letter D and the letter L to designate configuration. And so moving forward in our course, we're going to talk about each of these two systems in separate videos, starting with the Cahn-Ingold-Prelog system. So I'll see you guys in our next video.

Monosaccharide Configurations

So in this video, we're going to refresh your guys' memories of how to use the Cahn-Ingold-Prelog system to designate absolute configurations to chiral carbons using the letters R and S. The Cahn-Ingold-Prelog System is really just a synonym for R-S configuration.

Let's take a look at our sugar down below, glyceraldehyde. The chiral carbon is this carbon atom right here in the middle. Notice that the carbon at the top is not chiral because it has a double bond, or 2 bonds leading to the same chemical group of an oxygen atom. And the carbon at the bottom is also not chiral because it has two bonds leading to the same chemical group of a hydrogen atom. The only reason that the chiral carbon in the center is chiral is because it has 4 distinct chemical groups attached to it.

When we're trying to determine the configuration of this chiral carbon using the Cahn-Ingold-Prelog system and R-S configuration, these are the 3 steps that we really need to consider. The first is to assign priorities number 1 through 4 to each of the chemical groups that are attached to the chiral carbon. These assignments are based on the atomic numbers of the atoms in each chemical group. Looking at these 4 distinct pathways connected to this chiral carbon, we have a hydroxyl group, this entire group here, then this group down here, and, last but not least, this group to the left.

The oxygen atom has the highest atomic number and is assigned priority number 1. The carbon atoms in the blue and green groups have equal priority until you consider the substituents attached to those carbons. The carbon atom at the top has a double bond to an oxygen, whereas the carbon at the bottom only has one single bond to an oxygen, so the double bond has a higher priority. This pathway gets priority number 2, and the carbon atom down here gets priority number 3. That leaves the hydrogen atom with priority number 4.

In the second step, we determine if the configurations of priorities 1, 2, and 3 (ignoring 4), are arranged in a clockwise R configuration or a counterclockwise S configuration. After analyzing the priorities from 1 to 2, 2 to 3, and 3 back to 1, we see that it forms a counterclockwise pattern, which appears to be an S configuration.

The last and final step (step number 3) is only necessary if priority number 4 is on a wedge popping out of the page. Checking priority number 4, we see it's on a horizontal bond of the Fisher projection, which means it is indeed on a wedge. Therefore, even though this looks like an S configuration, because priority number 4 is on a wedge, this is actually an R configuration. So, this chiral carbon in the center is assigned an R configuration.

This video has outlined the steps we take to determine R-S configuration, which might be crucial as your professors could expect you to remember how to do this. Moving forward, we have some practice problems to help refresh your memories. Hopefully, this was a helpful refresher for you guys. In our next video, we'll talk about how to determine D-L configuration using Fisher's convention, so I'll see you guys there.

Monosaccharide Configurations

So now that we've refreshed our memories on how to designate RS configuration using the Cahn-Ingold-Prelog system, in this video, we're going to talk about how to designate DL configurations using Fisher's convention, which is actually more commonly used on carbohydrates since Fisher's convention was developed using carbohydrates. And so, Fisher's convention again is going to designate either D or L configurations relative to Glyceraldehyde enantiomers. And so, glyceraldehyde is just the name of one of the smallest and simplest sugars that exist. And again, enantiomers are just referring to non-superimposable mirror images.

And so, if we take a look down below at the image here in this box, notice that we're showing you the 2 Glyceraldehyde enantiomers. And so what's important to know is that the D configuration is going to be assigned to a chiral carbon when the hydroxyl group of that chiral carbon is pointing to the right, and the L configuration will be assigned to a chiral carbon when the hydroxyl group of that chiral carbon is pointing to the left of the Fischer projection. And so you can think L is for left. And so if we take a look down below at our Glyceraldehyde enantiomers, notice that this enantiomer over here has its hydroxyl group on its chiral carbon pointing to the right of the Fischer projection. And of course, if the hydroxyl group is pointing to the right, then we must assign a D configuration. And so that means that this glyceraldehyde enantiomer here is the D-Glyceraldehyde form and has D configuration.

Now notice looking at this enantiomer over here, the mirror image, its hydroxyl group on its chiral carbon is pointing to the left of the Fischer projection. And of course, a hydroxyl group pointing to the left corresponds with L configuration. And so this glyceraldehyde enantiomer over here is indeed the L-Glyceraldehyde form and has L configuration.

Now, one thing that's important to note here is that Fisher's convention will only properly work with standard Fisher projections. And recall that way back in our previous lesson videos, we covered standard Fisher projections. And so recall that standard Fisher projections will always have the carbonyl group on the top of the Fischer projection or towards the top of the Fischer projection. And so notice that with these glyceraldehyde enantiomers, the carbonyl group is hidden within the aldehyde group here, and so the carbonyl group is indeed on the top. And also recall that with standard Fisher projections, the longest carbon chain must be vertical. And so when we take a look at these Glyceraldehyde enantiomer over here, notice that the longest carbon chain is indeed vertical in both of these. So these are standard Fischer projections that are being shown. And most of the time, you will be given Fisher projections that are already in standard Fisher projection form, so that makes your job a little bit easier.

Now, usually under most circumstances that you'll encounter in your biochemistry course, D configurations are going to equal R configurations, and usually, L configurations are going to equal S configurations. But this is not always going to be true under every single circumstance. Again, this is going to be usually true under most circumstances that we will encounter. But if you want to be 100% safe, then all you really need to do is determine RS configuration separately from DL configuration.

Alright. So now that we know how to determine DL configurations, let's go ahead and determine the DL configurations for all of the chiral carbons in these 2 glucose enantiomers shown over here. And so if we take a look at this first chiral carbon right here, notice that its hydroxyl group is pointing to the right. And of course, an OH group pointing to the right corresponds with D configuration. And so that means that we can give this chiral carbon right here a D configuration. So we'll put a little D right next to it. Now moving on to the next chiral carbon right here, notice that its hydroxyl group is pointing to the left. And of course, a left-pointing hydroxyl group corresponds with L configuration. So we can go ahead and give this chiral carbon right here an L configuration.

And then notice that these next two chiral carbons both have their hydroxyl groups pointing to the right, which again corresponds with D configuration. So we can go ahead and assign both of these chiral carbons D configuration. So now moving on to the next glucose enantiomer over here, notice that its first chiral carbon right here has its hydroxyl group pointing to the left, which of course corresponds with L configuration. So we can go ahead and give this chiral carbon an L configuration. Notice that the next chiral carbon right here has its hydroxyl group pointing to the right, so it's going to have a D configuration, so we can put D over here. And then the next 2 chiral carbons have both of their hydroxyl groups, or have their hydroxyl groups pointing to the left, which corresponds with L configuration. So we can put 2 Ls here.

And so the real question is, how do we determine the configuration of an entire monosaccharide if it contains a mixture of configurations, a mixture of D and L configurations. Well, really, that's what these next two bullet points are addressing right here. And so a monosaccharide will have the same exact configuration as the configuration of the highest numbered chiral carbon, or in other words, to say the same exact thing, we can say that a monosaccharide's configuration is going to be that of the chiral carbon that is furthest from its carbonyl group. And so if we take a look at this glucose enantiomer right here, notice that its carbonyl group is way up here at the top in this aldehyde group. And so the chiral carbon that is furthest away from the carbonyl group is going to be this chiral carbon right down here. And so that means that the configuration of this entire monosaccharide is going to be the same as this chiral carbon that's furthest away, which is a D configuration. And so that means that this entire glucose enantiomer is the D-glucose enantiomer.

Now if we take a look at this other enantiomer over here, notice that its carbonyl group, again, is at the very top, here in this aldehyde group. And the chiral carbon that's furthest from this carbonyl group is this one right here. And so this one has an L configuration, which means that this entire monosaccharide here will also have the L configuration, so this will be L-glucose. And so, really, this, you'll be able to get a good hang of, how this works as you start to get more practice. So moving forward in our course, we will definitely be able to apply these concepts in practice problems.

Now, the last thing that I want to leave you guys off with is that unlike biological amino acids, which we know from way back in our previous lesson videos are usually in the L configuration, biological carbohydrates on the other hand, they tend to usually be in the D configuration. And so, the real reason for why life prefers L amino acids but it prefers D carbohydrates is unknown. And so there are some theories, but we are not going to cover those theories here. But what you do need to know is that life prefers D carbohydrates and life prefers L amino acids. And of course, there are some exceptions, but for the most part, these are the preferences. And so this concludes our lesson on Fisher's projection and how to determine DL configuration. And again, we'll be able to get some practice as we move forward in our course. So I'll see you guys in our next video.

Identify the D/L and R/S configuration of all of the following monosaccharides:

So this practice problem says, identify the DL and RS configuration of all of the following monosaccharides, and notice that we've got these 4 monosaccharides shown below. Now, at first glance, maybe this practice problem seemed kind of long and challenging because maybe you interpreted this problem as saying, determine the DL and RS configuration of every single chiral carbon that's present, but that is not what this practice problem wants us to do. Instead, it's asking us to determine the DL and RS configuration of the following monosaccharides as a whole. Recall from our last lesson video that an entire monosaccharide as a whole will take the same configuration as the one chiral carbon that's furthest away from the carbonyl group. We only need to determine 1 chiral carbon configuration to get our answer for each of these 4 monosaccharides below. Let's take a look at our first sugar over here and notice that this sugar is in standard Fischer projection. Its carbonyl group here, which is a ketone group, is as close to the top as it can possibly be. It is towards the top and the longest carbon chain is indeed vertical. Also, notice that this sugar here only has 1 chiral carbon, which makes our job much easier. That chiral carbon is right here. Notice that the hydroxyl group is pointing to the right of the Fischer projection, which means that this chiral carbon is taking a D configuration. We can put D-configuration up above and also recall from our last lesson video that usually, in most circumstances in our course, the D configuration is going to translate to R configuration. You could have also used the Cahn-Ingold-Prelog system on this chiral carbon to determine that it has R configuration. This is the answer to part A. Moving on to our next sugar here, notice that the carbonyl group is hidden in this aldehyde group at the top. The chiral carbon that's furthest away from this carbonyl group is this chiral carbon right here. Notice that the alcohol group of this chiral carbon is pointing to the left of the Fischer projection, which means that the chiral carbon has L configuration. We can put L up here at the top and also recall from our last lesson video that usually, in our course, L configuration will translate to S configuration. Again, you could have also used the Cahn-Ingold-Prelog system rules to determine that this chiral carbon has S configuration. This is the answer to part B. For part C, you can see that here we have our carbonyl group present as a ketone group, and the chiral carbon furthest away from this carbonyl group will be this carbon here. Its hydroxyl group is pointing to the right, meaning that it is going to have a D configuration. So we can put D configuration here, and as already mentioned, D configuration translates to R configuration in most circumstances here. Again, we could have used the Cahn-Ingold-Prelog system rules on this chiral carbon to determine that it has R configuration. This is the answer to part C. Lastly, for part D, notice that its carbonyl carbon atom is hidden in this aldehyde group at the top. The chiral carbon furthest away from the carbonyl group will be this chiral carbon here. Notice that the alcohol group of this chiral carbon is pointing to the left of the Fischer projection, which means that this chiral carbon has an L configuration. We can put L at the top. Again, usually in most circumstances, L configuration will translate to S configuration. We could have also used the Cahn-Ingold-Prelog system rules to determine that this chiral carbon has S configuration. These are the answers to our practice problem, concluding this practice. I'll see you guys in our next video.

How many D-stereoisomers are possible for a D-ketopentose?

• A. 4
• B. 8
• C. 24
• D. 2
• E. 16

Which TWO of the following traits apply to Fischer's Convention for naming stereoisomers?

a) Utilizes (+) and (-) designations.

b) Utilizes (D) and (L) designations.

c) Utilizes (R) and (S) designations.

d) Requires drawing the compound with aldehyde/ketone at the top.

e) Based on optical properties of the compound in solution.

f) Reflects stereochemistry at multiple centers.

So this practice problem is asking which 2 of the following traits applied to Fisher's convention for naming stereoisomers, and we've got these 6 potential answer options down below. And so what we need to recall from our previous lesson videos is that Fisher's convention refers to the D or L configurations. And really, it's the Cahn-Ingold-Prelog system that refers to R and S configurations. Therefore, we can cross off option C, and we can actually highlight option B as one of the correct answers. But recall, we're looking for 2 correct answers. So, there is another correct answer amongst these options. And so, what we also need to realize is that, we can eliminate option A because it says that Fisher's convention utilizes plus and minus designations. However, this is not something that we mentioned in our previous lesson videos, and this is not true. And so you might recall from way back in your old organic chemistry courses that the plus and the minus designations are indicating the optical activity of the molecule, but Fisher's convention does not convey optical activity, so plus and minus is not going to be correct. And this also means that we can eliminate option E because it suggests again that Fisher's convention is based on optical properties of the compound in solution, but we just mentioned that this is not the case when it comes to Fisher's convention. So, now, we're left with either option D also being correct or also option F also being correct. Now, when we look at option F, notice that it says that Fisher's convention reflects stereochemistry at multiple centers or at multiple chiral centers. But of course, we know from our previous lesson videos that this is not true. Fisher's convention will give a D or an L configuration, but that D or L configuration only reveals the stereochemistry of 1 center, not multiple centers. As written, option F is incorrect. And so, of course, this means that option D must be the correct answer here, and it says that Fisher's convention requires drawing the compound with the aldehyde or ketone at the top. And so really, option D is just a reflection of making sure that the Fischer projection is in standard Fischer projection form. Thus, this is definitely something that must apply to Fisher's convention. We can only use Fisher's convention when the structure is in standard Fischer projection. So the two correct answers for this practice problem are options B and D. And so that concludes this practice problem, and I'll see you, guys, in our next video.

How many possible epimers of D-glucose exist?

• A. 1
• B. 2
• C. 4
• D. 6
• E. 8
• F. 16

Here’s what students ask on this topic:

What is the Cahn-Ingold-Prelog system for designating monosaccharide configurations?

The Cahn-Ingold-Prelog system, also known as the R/S system, is used to designate the absolute configurations of chiral carbons in monosaccharides. This system assigns priorities to the four substituents attached to the chiral carbon based on their atomic numbers. The steps include: 1) Assigning priorities from 1 to 4, 2) Determining the order of priorities 1, 2, and 3 (clockwise for R and counterclockwise for S), and 3) Checking if the lowest priority group (4) is on a wedge. If it is, the configuration is reversed. This system helps in understanding the spatial arrangement of atoms around the chiral center.

How do you determine D and L configurations using Fischer projections?

Fischer projections are used to determine D and L configurations of monosaccharides. In this system, the configuration is based on the orientation of the hydroxyl group (-OH) on the chiral carbon furthest from the carbonyl group. If the hydroxyl group is on the right, the molecule is in the D configuration; if it is on the left, it is in the L configuration. This method is particularly useful for carbohydrates, as it was developed using glyceraldehyde enantiomers. Fischer projections must be in standard form, with the carbonyl group at the top and the longest carbon chain vertical.

What is the relationship between D/L and R/S configurations in monosaccharides?

In monosaccharides, D configurations usually correspond to R configurations, and L configurations usually correspond to S configurations. However, this is not always the case under all circumstances. The D/L system is based on the orientation of the hydroxyl group in Fischer projections, while the R/S system is based on the Cahn-Ingold-Prelog priority rules. To be 100% accurate, it is recommended to determine R/S and D/L configurations separately.

Why are D configurations more common in biological carbohydrates?

Biological carbohydrates tend to be in the D configuration, while biological amino acids are usually in the L configuration. The exact reason for this preference is unknown, though there are several theories. This preference is significant in biochemistry because it affects how these molecules interact with enzymes and other biological molecules. Understanding this preference helps in studying metabolic pathways and the synthesis of biomolecules.

How do you assign priorities to substituents in the Cahn-Ingold-Prelog system?

In the Cahn-Ingold-Prelog system, priorities are assigned to substituents based on their atomic numbers. The higher the atomic number, the higher the priority. If two substituents have the same atomic number, the next atoms in the substituents are compared. Double and triple bonds are treated as if the atoms were duplicated or triplicated. This method ensures a consistent way to determine the spatial arrangement of atoms around a chiral center.