Anomer: Videos & Practice Problems

Anomers are cyclic sugars that differ in the configuration of their anomeric carbons, which become chirality centers during monosaccharide cyclization. The two configurations are alpha and beta. In the alpha anomer, the hydroxyl group on the anomeric carbon points down, opposite the highest numbered carbon, while in the beta anomer, it points up, on the same side as the highest numbered carbon. Understanding these configurations is crucial for recognizing the structural differences in cyclic sugars like D-glucose.

Anomer

So now that we're familiar with how monosaccharides cyclize, in this video we're going to introduce anomers. Monosaccharide cyclization also generates alpha and beta anomers, but what in the world are anomers? Well, anomers are defined as cyclic sugars that differ only in the configurations of their anomeric carbons. Recall from our previous lesson videos, we already know that the anomeric carbon is the only ring carbon that's attached to 2 oxygen atoms, and the anomeric carbon can also be defined as the carbon atom that used to be the carbonyl carbon before cyclization. We know from our previous lesson videos that upon monosaccharide cyclization, the anomeric carbon is formed, and the anomeric carbon becomes part of either a hemiacetal or a hemiketal. But what we have not yet mentioned is that upon monosaccharide cyclization, the anomeric carbon becomes a chirality center, meaning that it's going to have 2 possible configurations. The first is the alpha configuration, and the second is the beta configuration. This is what leads to the alpha anomer and the beta anomer depending on the configuration of the anomeric carbon.

So before we actually define the alpha and beta anomers, let's take a look down below at our image and notice over here on the far left we're starting with a linear or an open-chain sugar, D-glucose, which has an aldehyde group up here on carbon number 1, and it has this highlighted hydroxyl group here on carbon number 5. We've got our muscle man here bending our linear glucose so that these 2 groups are in close proximity. Notice the hydroxyl group here on C5 is in close proximity to the aldehyde group here on carbon number 1. We know from our previous lesson videos that the alcohol group is going to act as a nucleophile and attack this carbonyl group. But this alcohol group can actually attack this carbonyl group in 2 different ways. This leads to these 2 different pathways and these 2 different results that we see here. This alcohol group can either attack the carbonyl group from the top side of the carbonyl or could attack from the bottom side of the carbonyl.

If the alcohol group attacks from the top side, then what we'll get is this anomer that we see here, which is the alpha anomer or alpha-D-Glucopyranose. What exactly is this alpha anomer? This is going to be when the anomeric carbon's hydroxyl group is on the opposite side of its highest numbered carbon. Take a look at this alpha anomer down below, its anomeric carbon is this C1 carbon here. Notice that its alcohol group is going down below the plane of this ring here, pointing in a downwards fashion, and notice that the highest numbered carbon atom is carbon number 6 here and carbon number 6 is pointing in an upwards fashion. Thus, we have carbon number 6 pointing upwards and the alcohol group of the anomeric carbon pointing downwards. They are pointing in opposite directions. With alpha here, the hydroxyl groups, of the anomeric carbon is reaching down for the ants, which helps me remember the alpha anomer.

On the other hand, if this hydroxyl group doesn't attack from the top, instead, it attacks from the bottom, then what we'll get is this other anomer over here, and this is the beta anomer or beta-D-Glucopyranose. The beta anomer is going to be when the anomeric carbon's hydroxyl group, instead of being on the opposite side, is now going to be on the same side of its highest-numbered carbon. This time the hydroxyl group is going upwards, pointing in the same direction as the highest numbered carbon, carbon number 6. Carbon number 6 and the hydroxyl group are both on the same side. What helps me remember that the beta confirmation is on the same side is that when you write beta, notice that the two bumps of this beta are on the same side as this pole here.

One thing to point out here is that these two anomers, the alpha anomer and the beta anomer, are absolutely not mirror images of one another. As close as they might seem, they are not mirror images, which is why we have this broken mirror here to remind you of that. Again, you can see that this alcohol group of the anomeric carbon is reaching down for the ants, so it's going to be the alpha form, and this alcohol group of the anomeric carbon is on the same side as the highest numbered carbon, so it's going to be in the beta form. It's important to be able to distinguish and recognize these two different anomers that can exist for cyclic sugars.

That concludes our introduction to anomers, and we'll be able to get some practice applying these concepts as we move forward in our course. So, I'll see you guys in our next video.

Anomer Example 1

Alright. So here we have an example problem that's asking which of the following molecules are anomers? We've got these 5 potential answer options down below. Now recall from our last lesson video that anomers are literally cyclic sugars that are identical in every way except for the configuration of the anomeric carbon. When we looked at option A, comparing molecule 1 with molecule 2, we first compared the anomeric carbons. The anomeric carbon is going to be the only carbon atom that is bound to 2 oxygen atoms. So for sugar 1, the anomeric carbon is this carbon here, and for sugar 2, the anomeric carbon is this carbon here. Notice that the alcohol group on the anomeric carbon is pointing in the same direction away from the highest numbered carbon. This means that both of these are in the alpha configuration. But for them to be anomers, they must have opposite configurations. So these 2 are not anomers of each other. So we can go ahead and cross off option A.

While we're looking at molecule number 1, let's move on to option C, which also looks at molecule 1 and compares it with molecule 5. When we look at the anomeric carbons of both of these sugars, notice they are going in opposite directions. Sugar 1 is in the alpha configuration, and sugar 5's alcohol group on the anomeric carbon is going in the same direction as the highest numbered carbon, making a beta configuration. So far, comparing these 2, they seem to be anomers. But remember, the only difference between these two sugars must be in the configuration of the anomeric carbon for them to be considered anomers. Notice these two sugars differ from each other in more than just the configuration that we've already noted. The carbon over here has its hydroxyl going up, whereas this carbon has its hydroxyl going down. So these two carbons differ from each other in more than just the configuration of the anomeric carbon, which means they are not anomers of each other, and we can go ahead and cross off option C.

Again, while we're looking at molecule 1, let’s move on to option E, which compares molecule 1 to molecule 4. Notice that molecule 1 is a pyranose with a 6-membered ring, whereas molecule 4 is a furanose with a 5-membered ring. So already, there's no way these 2 can be anomers of each other. So we can go ahead and cross off option E. Now, we're between either option B or option D, and notice option D, comparing molecule 3 here to molecule 4. When we look at the anomeric carbons, notice the anomeric carbon is here and here. Notice that they both have the same configuration of beta. So, again, for them to be anomers, they must have opposite configurations. They have the same configuration, which means they are not anomers, which of course leaves us with option B, molecule 2, and molecule 5. Looking at molecule 2 and molecule 5, notice that their anomeric carbons are in opposite configurations, this one is reaching down for the ants, so it's going to be alpha, and this one here is reaching up, to the same side as the highest numbered carbon, so the same side is going to be beta. Comparing the rest of both of these molecules, the rest of this and the rest of this are identical to each other. The only way these 2 differ from each other is in the configuration of the anomeric carbon, and that's what makes them anomers. So, we can go ahead and indicate that the correct answer for this problem is option B.

That concludes this practice problem. I'll see you guys in our next video.

The ___/___ configuration of a monosaccharide is determined by the ________________ of the chiral carbon furthest from the carbonyl group, while the ___/___ anomers are determined by _______________ of the anomeric carbon.

• A. D ; L ; conformations ; R ; S ; configuration.
• B. R ; S ; conformation ; α ; β ; configuration.
• C. D ; L ; configuration ; α ; β ; conformation.
• D. D ; L ; configuration ; α ; β ; configuration.
• E. D ; L ; conformation ; α ; β ; conformation.

Answer: (D)

Answer the following questions regarding the following cyclic monosaccharide shown below:

A) Clearly label the hemiacetal carbon.
B) The monosaccharide is a(n) _________ anomer.
a) Alpha (α).
b) Beta (β).
C) Draw the opposite anomer.
D) Draw the α stereoisomer that differs in the arrangement of substituents at C2.

Alright. So here we have a 4-part practice problem broken up into part a, part b, part c, and part d way down below. It says, answer the following questions regarding the following cyclic monosaccharide shown below, which is right here. And so part a says, to clearly label the Hemiacetal Carbon and so what we need to realize here is that the Hemiacetal Carbon is just going to be the anomeric carbon itself. And so, of course, we know that the anomeric carbon is going to be the only carbon atom that's bound to 2 oxygen atoms. And so, taking a look at this cyclic sugar over here, notice that the anomeric carbon is right here and it's bound to the ring oxygen and the oxygen atom of the hydroxyl group. And so if we want to clearly label this atom, we can go ahead and put an arrow here like this and just label it the Hemiacetal or, in other words, the anomeric, carbon. And so, we can move on to part b now, which says that the monosaccharide is a blank anomer, and the answer options are either alpha or beta anomer. And so when we want to consider the configuration of the anomer, of course, we're going to look at the anomeric carbon. And so notice that the anomeric carbon has its hydroxyl group pointing upwards. And when it's pointing upwards in the same direction as the highest numbered carbon, So if we were to number these carbons, of course, we would want the anomeric carbon to have the lowest possible number. So this would be carbon, this carbon would be carbon number 1, carbon 2, carbon 3, carbon 4, carbon 5, and then, of course, carbon number 6 would be this one up here. So, now, we can see that the hydroxyl group is pointing upwards in the same direction as the highest numbered carbon. And so this means that it's going to have the beta configuration since again, the bumps here of the beta are on the same side just like the hydroxyl and the highest numbered carbon are on the same side. So, what we can say is that the answer here is going to be B beta is the correct answer for part b. And so now, moving on to part c, it wants us to draw the opposite anomer. And so if we want to draw the opposite anomer, the opposite anomer is only going to differ in the configuration of the anomeric carbon. So that means that we just need to draw the same exact structure that we see here. And so when we do that, we get this structure here; and again, we only wanna change the configuration of the anomeric carbon. So we're only gonna change the direction that the alcohol group is pointing at. Instead of having the alcohol group pointing upwards in the same direction as the highest numbered carbon, we want it to be going downwards and the highest numbered carbon is going upward. So, that makes this the alpha anomer, and that's it for part c, and so, moving on to part d, notice that it wants us to draw the alpha stereoisomer that differs in the arrangement of the substituents at C2, which is the C2 carbon. And so, if we want the alpha stereoisomer, that means that we want the one that we just drew. So, all we need to do is, copy the same exact structure that we have here and we want to change the substituents at carbon number 2, the C2 carbon, and so, recall that the anomeric carbon here is carbon number 1 and so the C2 carbon is going to be this carbon right here. So that means that we want to change the arrangement here. So instead of having the hydroxyl group going up on the C2, we want the hydroxyl group going down. So we can go ahead and put the hydroxyl group here, put it in green, hydroxyl group going down here and of course the hydrogen atom we can have going up. And so really this is the only thing that we needed to do to change this position here at C2, and that is the answer for part d. And so that concludes our 4-part practice problem, and I'll see you guys in our next video.

Here’s what students ask on this topic:

What are anomers in biochemistry?

Anomers are a type of stereoisomer found in cyclic sugars that differ in the configuration around the anomeric carbon. The anomeric carbon is the carbon that was the carbonyl carbon (either an aldehyde or ketone) before the sugar cyclized. During cyclization, this carbon becomes a new chirality center, leading to two possible configurations: alpha (α) and beta (β). In the α-anomer, the hydroxyl group attached to the anomeric carbon is on the opposite side of the ring from the highest numbered carbon. In the β-anomer, the hydroxyl group is on the same side as the highest numbered carbon. Understanding anomers is crucial for recognizing structural differences in sugars like D-glucose.

How do alpha and beta anomers differ?

Alpha (α) and beta (β) anomers differ in the orientation of the hydroxyl group attached to the anomeric carbon. In the α-anomer, the hydroxyl group on the anomeric carbon points down, opposite to the highest numbered carbon in the ring. In contrast, in the β-anomer, the hydroxyl group points up, on the same side as the highest numbered carbon. This difference in configuration occurs because the hydroxyl group can attack the carbonyl carbon from either the top or bottom side during cyclization, leading to these two distinct forms.

What is the anomeric carbon in a sugar molecule?

The anomeric carbon in a sugar molecule is the carbon that was originally the carbonyl carbon (either an aldehyde or ketone) before the sugar underwent cyclization. During the cyclization process, this carbon becomes a new chirality center, meaning it is bonded to four different groups. The anomeric carbon is unique because it is the only carbon in the ring structure that is attached to two oxygen atoms. Its configuration determines whether the sugar is in the alpha (α) or beta (β) anomeric form.

Why is the concept of anomers important in biochemistry?

The concept of anomers is important in biochemistry because the different configurations (alpha and beta) of the anomeric carbon can significantly affect the properties and biological functions of sugars. For example, the α and β forms of glucose have different reactivities and can interact differently with enzymes and other biomolecules. This distinction is crucial for understanding carbohydrate metabolism, enzyme specificity, and the structural roles of polysaccharides in biological systems.

How can you identify the alpha and beta anomers of D-glucose?

To identify the alpha (α) and beta (β) anomers of D-glucose, you need to look at the orientation of the hydroxyl group attached to the anomeric carbon (carbon 1) in the cyclic form. In the α-anomer of D-glucose, the hydroxyl group on the anomeric carbon points down, opposite to the highest numbered carbon (carbon 6). In the β-anomer, the hydroxyl group on the anomeric carbon points up, on the same side as the highest numbered carbon. This difference in orientation is due to the way the hydroxyl group attacks the carbonyl carbon during cyclization.