Glycosidic Linkage - Online Tutor, Practice Problems & Exam Prep

A glycosidic linkage forms between the anomeric carbon of one monosaccharide and another monosaccharide through dehydration, resulting in a disaccharide. Hydrolysis reverses this process, splitting the disaccharide back into monosaccharides by adding water. Glycosidic linkages can be classified as alpha or beta based on the orientation of the anomeric hydroxyl group. For example, maltose has an α-1,4 linkage, while cellobiose has a β-1,4 linkage. Sucrose features both an α-1 and β-2 linkage due to its two anomeric carbons. Understanding these linkages is crucial for grasping carbohydrate structure and function.

Glycosidic Linkage Formation Concept 1

In this video, we're going to talk about the glycosidic linkage. Now our glycosidic linkage formation—this is just an acetyl or acetyl bond, so depending on how you want to pronounce it—between a sugar's anomeric carbon, which remember is carbon number 1, and another monosaccharide. We're going to say it's formed via dehydration, which is the loss of water. And when we're looking at the equation for a glycosidic linkage formation, we can just say it as monosaccharide 1 plus monosaccharide 2 will give me what we call a disaccharide. So remember, a disaccharide is two monosaccharides together, and what links them together is the glycosidic linkage.

So, if we take a look here, we're going to say we have Alpha D glucose, Alpha D glucose. The anomeric carbon, which is carbon number 1 for the monosaccharide on the left, is going to interact with the hydroxyl group (the OH group) of Carbon number 4 of the other monosaccharide unit. By their interactions, we're going to lose water. So, if we come over here, we've lost water, so this oxygen now needs to make up the bond that it just lost. This anomeric carbon needs to replace the bond it just lost.

So what they do is they connect together. Here, this represents our glycosidic linkage. This is the bond that connects together my two monosaccharides to one another, transforming my individual monosaccharides into a disaccharide.

Glycosidic Linkage Formation Example 1

Here it says, "Provide the structure of the disaccharide formed when the hydroxyl groups of the highlighted carbons undergo a dehydration reaction." Alright. So here, we'll treat the monosaccharide on the left; this is our anomeric carbon here, and what we're going to say here is for these two OH groups to interact, they need to be next to each other. So, we're going to need to actually flip this second monosaccharide over so that the OH groups are next to each other. So, we're going to have to do a little bit of redrawing here. So we still have this here. And here, we're just going to worry about how the rings interact, and then we'll come over here and redraw it as our final answer when it's cleaned up. So again, I'm flipping this, so now this position, which is also position 1, I'm going to draw it over here. Okay. So there goes the OH. Alright. So 1, 2, 3, 4, 5, and 6. Again, I'm flipping this. So I'll put the oxygen here. Right. So 1, 2, 3, 4, 5, and 6. So I kind of just flipped it over like that. We have the interaction, which causes a loss of water. So, at the end, we're going to form our glycosidic linkage. So, we're going to come over here and clean it up. Here goes the glycosidic linkage. Alright. So this is 1, 2, 3, 4, 5, 6 here, which is 1, 2, 3, 4, 5, 6 here. So we just gotta draw things in the right positions. On carbon number 2, the OH is down, then the OH is up, then the OH is down, and then we have CH₂OH. On the other ring, which we have numbered here, let's redraw this. So we have this oxygen. Okay. So then let's see. 2, which is this 2 here, the OH is down. And, again, numbering this 1, 2, 3, 4, 5, 6. On 3, the OH is up. On 4, the OH is down. And on 5, the CH₂OH is up. So this is how we would show these two monosaccharides. And in this one, both anomeric carbons, both position ones, are what is connecting together to make the glycosidic linkage. It doesn't always have to be 1→4, and this one we're seeing is between 1→1. But the same process still is followed. We have a dehydration reaction, so we have the loss of water to make the glycosidic linkage.

Hydrolysis of Glycosidic Linkage Concept 2

In this section, we talk about the hydrolysis of a glycosidic linkage. Now, we're going to say, under this reaction, a glycosidic linkage is hydrolyzed into 2 monosaccharides. Recall that hydrolysis itself is a reaction that breaks down a molecule through the addition of water. Here, both sugar carbons regain their OH or hydroxyl groups as a result of this hydrolysis.

So, here we have our disaccharide, which is 2 monosaccharides connected to one another through this glycosidic linkage. Here, we're going to have the addition of water. Water is going to come in and actually help separate these from each other. As a result, we're going to have this anomeric carbon, carbon number 1, regaining its OH group. The oxygen that was already there is still there. We'll give it to this carbon, and then it gains an H. We're adding water: one gets the OH, and one gets the H, but the end result is that both carbons at the end regain their OH group.

Splitting this disaccharide has created for us alpha-D-glucose and beta-D-glucose as our 2 monosaccharide units. Alright, so just remember, hydrolysis is undoing the glycosidic linkage through the addition of water.

Hydrolysis of Glycosidic Linkage Example 2

Here it says, provide the monosaccharide units produced by hydrolysis of the following disaccharide. So here is our anomeric carbon, carbon number 1. And the glycosidic linkage is this right here. We're gonna break it apart. We'll break these 2 free of one another through the addition of water, with a little bit of acid catalyst, H⁺. What happens here is we're gonna get this monosaccharide being recreated by itself. These OH's are in the same position. This CH₂OH is in the same position. Now we have the anomeric carbon having its OH group back. Plus we're gonna draw this monosaccharide, CH₂OH, OH, OH, OH, CH₂OH. This oxygen is still there, so it gets to keep it. And then it gains a hydrogen so that we have an OH at the end. So, through the hydrolysis of this disaccharide we've created 2 separate monosaccharide products. Both of the carbons that were involved in the glycosidic linkage each have regained an OH group.

Alpha vs Beta Linkages Concept 3

We know that a glycosidic linkage connects two different monosaccharides to one another, but there's also an alpha version and a beta version. Now, here we're going to say this type of linkage is always defined by the linked anomeric hydroxyl group. Remember, your anomeric hydroxyl group is connected to the anomeric carbon, which is carbon number 1 of one of your monosaccharides. We're going to say here alpha and beta linkages are defined in the same way as cyclic monosaccharides. And we’re going to say the exception here is sucrose. It possesses two linked anomeric hydroxyl groups, so both must be named. If we take a look at the different types of glycosidic linkages, let's take a look at maltose. In maltose, we would say that this CH₂OH is up, but then the bond here of the anomeric carbon is pointing down. It's on the opposite side. Based on what we know about cyclic monosaccharides, opposite sides mean that it is alpha. So this would be an alpha-1,4 glycosidic linkage. Alpha because it's pointing down, and 1,4 because it's the anomeric carbon, carbon 1, and on the other monosaccharide, carbon number 4, that are connecting together to make my linkage.

Next, we have cellobiose. If we look here, CH₂OH is pointing up, and the bond that is connected is part of the linked anomeric hydroxyl group; it's also pointing up. We said that if your OH and your CH₂OH are on the same side with each other, that would be beta. The same applies here. So this is a Beta-1,4 linkage for this particular disaccharide. Next, we don't have a 1,4 here, we have a 1,6. So the same rules apply. We look at the anomeric carbon. We look at its linked hydroxyl group. It is pointing down. CH₂OH is pointing up. They're on opposite sides. That would mean that it is alpha. So here, we're going to say that amylopectin is going to be an alpha-1,6 linkage because the carbons that are connecting the glycosidic bond or glycosidic linkage are carbons number 1 and carbon number 6.

Finally, we have sucrose. Remember, sucrose is different. We have two anomeric carbons here. So this is the anomeric carbon of this 6-membered ring. And technically here, this is the anomeric carbon of this 5-membered ring. If we take a look here, we would say that this linked hydroxyl group is pointing down and this CH₂OH is pointing up. They are on opposite sides of each other, so that would be alpha. And then we say that this anomeric carbon, we're going to say this is carbon 2 in this case. If we look at it this way, where it’s linked hydroxyl group, anomeric hydroxyl group is pointing up, and this CH₂OH group is also pointing up, they're both pointing in the same direction, the same side, that would be beta. So here, we'd say sucrose is an Alpha-1,Beta-2 Glycosidic Linkage. So just remember, for the most part, we're following the same rules that we know of in terms of cyclic monosaccharides. With sucrose, it’s just that now there are two anomeric carbons, so you have to name them both: Alpha-1,Beta-2 glycosidic linkage for sucrose.

Alpha vs Beta Linkages Example 3

In this example question, it says, "Melobios represents a disaccharide that is several magnitudes sweeter than table sugar. Determine the type of glycosidic linkage connecting its 2 monosaccharide units." Alright. So, here is our glycosidic linkage. This is carbon number 1. If we look at the other ring, this is the anomeric carbon for this ring 1, and then we count towards 2, 3, 4, 5, and 6. So the connection is between the anomeric carbon of this top ring and carbon number 6 of the second ring. So we know that this is a 𝛼1,6 linkage. It is not sucrose, so we don't have to pay attention to both of the carbons connected. It's not going to be a situation of alpha 1 and beta 2 and none of that. Now we just have to determine if this is alpha or beta in nature. If we look, we would say that this linked anomeric hydroxyl group is pointing down. The CH2OH group that is on the same ring is pointing up. They are on opposite sides. And based on what we talked about when it comes to monocyclic sugars or cyclic monosaccharides, if CH2OH and the linked anomeric hydroxyl groups are on opposite sides, that means it's alpha. So we would say that it is an 𝛼1,6 linkage, which is the type of glycosidic linkage for this particular disaccharide.

Lactulose represents a man-made disaccharide that possesses a b-1,4 glycosidic linkage. Determine the two monosaccharide units created from its hydrolysis.

• A.
• B.
• C.

Here’s what students ask on this topic:

What is a glycosidic linkage and how is it formed?

A glycosidic linkage is a type of covalent bond that connects two monosaccharides to form a disaccharide. It forms between the anomeric carbon (carbon 1) of one monosaccharide and a hydroxyl group (OH) of another monosaccharide. This process occurs through dehydration, where a water molecule (H₂O) is removed. The resulting bond can be either an alpha (α) or beta (β) linkage, depending on the orientation of the anomeric hydroxyl group. For example, in maltose, the linkage is α-1,4, while in cellobiose, it is β-1,4.

What is the difference between alpha and beta glycosidic linkages?

The difference between alpha (α) and beta (β) glycosidic linkages lies in the orientation of the anomeric hydroxyl group. In an α-glycosidic linkage, the hydroxyl group attached to the anomeric carbon is positioned on the opposite side of the CH₂OH group of the monosaccharide. In a β-glycosidic linkage, the hydroxyl group is on the same side as the CH₂OH group. For example, maltose has an α-1,4 linkage, while cellobiose has a β-1,4 linkage.

How does hydrolysis break a glycosidic linkage?

Hydrolysis breaks a glycosidic linkage by adding a water molecule (H₂O) to the disaccharide. This reaction splits the disaccharide into its two monosaccharide components. During hydrolysis, the anomeric carbon of one monosaccharide regains its hydroxyl group (OH), and the other monosaccharide gains a hydrogen atom (H). This process effectively reverses the dehydration reaction that formed the glycosidic linkage, resulting in the separation of the monosaccharides.

What are some examples of glycosidic linkages in disaccharides?

Examples of glycosidic linkages in disaccharides include:

• Maltose: α-1,4 glycosidic linkage between two glucose molecules.
• Cellobiose: β-1,4 glycosidic linkage between two glucose molecules.
• Sucrose: α-1,2 glycosidic linkage between glucose and fructose, with an α-1 linkage from glucose and a β-2 linkage from fructose.
• Lactose: β-1,4 glycosidic linkage between galactose and glucose.

Why is understanding glycosidic linkages important in carbohydrate chemistry?

Understanding glycosidic linkages is crucial in carbohydrate chemistry because these linkages determine the structure and function of carbohydrates. The type of glycosidic linkage (α or β) affects the digestibility and biological activity of the carbohydrate. For instance, humans can easily digest α-1,4 linkages in starch but cannot digest β-1,4 linkages in cellulose. Additionally, glycosidic linkages play a key role in the formation of complex carbohydrates and glycoconjugates, which are essential for various biological processes, including cell signaling and energy storage.