Disaccharide - Online Tutor, Practice Problems & Exam Prep

A disaccharide consists of two monosaccharides linked by an O glycosidic bond, formed through a condensation reaction that releases a water molecule. The type of linkage, whether alpha or beta, influences the disaccharide's properties. For example, cellobios is a disaccharide formed from two D glucose molecules with a beta 1,4 linkage. Once formed, disaccharides do not undergo mutarotation, as their structure is fixed. Understanding these linkages is crucial for studying carbohydrates and their functions in biological systems.

Disaccharide

What's up? Guys, by now you should know quite a bit about monosaccharides. But in this video, I want to take a step further and define what a disaccharide is. So let's get going with that. Guys, a disaccharide is very simply going to be 2 monosaccharides that are linked by an O-glycosidic linkage in the middle. Remember that an O-glycosidic bond would just be at the anomeric position and oxygen attached to an R. Well, if you have that where the r group is actually a second sugar, that's called a disaccharide. Now it doesn't matter whether it's an alpha linkage or a beta linkage, We'll get more into that in a little bit. That's just going to change the type of disaccharide it is, but disaccharides exist in multiple different forms that are both alpha and beta. Cool?

So guys, the exact reaction that brings these two sugars together into one disaccharide is called condensation. And guys, the reason that we call it a condensation reaction is because back when we learned about the definition of condensation, what did it always mean? Condensation always means that you take 2 molecules and you turn them into 1 bigger molecule with the release of water. You always have to release one water molecule to count as a condensation and that's exactly what happens. And the end product is that we form an acetal linkage. Okay? Remember we talked about acetals. I'm going to show you guys what the acetal looks like here.

Okay? Now one thing that you guys should just keep in mind about disaccharides is that once you form that disaccharide, those sugars are locked into position. So the typical type of mutarotation, epimerization that might happen when you have a single sugar that cyclizes doesn't really happen anymore for disaccharides. So what I'm trying to say here is that once you've formed your disaccharide, it's kind of stuck in that position. It's not just going to go from alpha to beta and beta to alpha by itself. Okay? Cool.

So let's just break down each step of the reaction. Let me explain this diagram to you. So guys, first of all, we need 2 sugars. So in this case, I decided to bring 2 molecules of D-glucose together to form a disaccharide. But guys, I could have used pretty much any monosaccharide I wanted. This is just a very common example and it's a very easy example to look at, but many different the multiple forms or multiple types of disaccharides exist using different types of sugars and using different types of linkages. So here, I'm just giving you an example.

So let's say that we wanted to form a beta-14 linkage between these 2 monosaccharides. Well, what would that even mean? Why would we call it a beta 14? Well guys, first of all, beta always stands for the first sugar and the relationship of the oxygen to the stereo descriptor carbon. So remember guys that this would be my stereo descriptor over here. It's facing up and in this case, this O because it's I'm using D-glucose, this O is also facing up, so that means that these are cysts to each other, so this would be beta. You guys remember that? Now it doesn't have to look like this, but this is just the one that we're using for right now.

Okay? So this is a beta D-glucose and notice that what I'm trying to do is I'm trying to form a linkage between the first carbon of my sugar 1 and the 4th carbon of sugar 2. And the way that we would count these guys is just like any cyclic sugar. We always count the anomeric position as 1 and then go around in a clockwise way. So then you would get 4 over here. Does that make sense? So if we were to bring these 2 alcohols together, the alcohol on 1 and the alcohol on 4, what we're going to get is the loss of water. And the reason is because if you're trying to make an acetal linkage, all you really need is one O, right? So that means that, let's say this is the O that stays, which it is. Okay, actually that wouldn't be the O that stays. I'm sorry. So let's say this is the O that leaves, let's say this is the H that leaves and let's say that this is the H that leaves. The O that stays is this one, right?

This is the one and we can go through the mechanism later but that O would wind up attaching to the carbon and notice that what we get as a byproduct is 1 O and 2 Hs leaving. So we would get the condensation reaction and the release of 1 water molecule. Okay? So this condensation reaction would then link these 2 together and notice that what we would now get in terms of this linkage is we would get what we call a beta-14 because once again, the linkage is going in the same direction as the stereo descriptor carbon and it's between carbon 1 of the first one and carbon 4 of the second one.

Okay. Now notice that 2 functional groups are present on this disaccharide. One, we have an acetal. Why do we call this an acetal? Because remember guys that the general formula for an acetal is that you have OR and OR connected to the same carbon. It's like a diether. Is that a diether? Absolutely. This is an acetal, but notice that the anomeric position of the other sugar in this case is still exposed. It still has an OH on it. So that means that in this specific disaccharide, I would have both an acetal functional group and a hemiacetal functional group present on the same molecule. This is going to be important later, but for right now, I'm just drawing your attention to it.

Okay. Now lastly, we have this name of cellubios. So you might be thinking, well Johnny, am I supposed to memorize that? Maybe. It turns out that some professors and some textbooks and homeworks want you to know a few of the most important disaccharides. You don't need to know all of them, but you might need to know a few important ones, which I'm going to go ahead and talk about in a second. But for right now, what you should know or what you can just take your take-home message from this is that the disaccharide called Cellubios would specifically be the disaccharide that's formed from 2 D-glucose molecules making a 1-4 linkage. And if you were to change any part of that, it would not be called Cellubios anymore. If you were to make it an alpha-1-4 linkage, where now it's facing away from the stereo descriptor, or if you were to change one of the glucoses into a fructose, all those changes would have massive impacts on the disaccharide. It would change the disaccharide completely and it would also change the name. Okay. So once again, cellubios is just the example that I'm giving for probably the easiest disaccharide to make, which is 2 D-glucose with a beta-1-4 linkage.

Okay. So, in the next video, I'm going to talk more about important disaccharides t

Identify the following disaccharides as reducing sugars (RS) or non-reducing sugars (NS)

• A. maltose = RS; sucrose = RS; lactose NS
• B. maltose = RS; sucrose = NS; lactose NS
• C. maltose = RS; sucrose = NS; lactose RS
• D. maltose = NS; sucrose = RS; lactose NS

Draw a theoretical acid-catalyzed Fischer Glycosidation mechanism for the condensation of D-Glucose into cellubiose. (Note:This reaction would lead to very poor yields of cellubiose- can you hypothesize why?)

Draw a theoretical acid-catalyzed Fischer Glycosidation mechanism for the condensation of D-glucose, that's 2 D-glucose molecules into cellobiose. Note this reaction would lead to actually poor yields of cellulose. Can you hypothesize why? First things first, let's just get this mechanism down on paper, and then we can discuss the theory of why this wouldn't work so well. What do we know about Fischer Glycosidation? It's an acid-catalyzed mechanism. So, let's just go ahead and add some acid in here. I'm going to go ahead and use H3O+. What we're trying to do is to remove this O2. According to the mechanism, we're trying to get rid of that O2 and get the new OR to attach. Now, the new OR group usually is just like a methanol, but in this case, it's going to be an entire sugar. I'm going to get that whole sugar to attack. Let's go ahead and draw out, step-by-step, what the mechanism would be. So, oxygen grabs the H. We're going to protonate, and let's go ahead and draw this out. You guys can pause if you are having a hard time following along. Here, I would have a proton and a positive charge, and this is where I can now make my good leaving group and kick it out using my O. What that's going to give me is a new sugar derivative, which is actually my intermediate. It's called my oxocarbenium, which is the most important intermediate of this chapter because we only react at the anomeric position and no other positions due to this resonance-stabilized intermediate with the O. The resonance structure would be either a plus on the O or a plus on the carbon. We want something other than just methanol or some random alcohol to attack; what we want to attack is the 4-position of my second sugar. Cellubiose would be a beta-1-4 linkage. I'm going to use this version of the intermediate, which is the easiest one to work with and easy to visualize that this O can now come from the top and attack that positive. Cool, now what I'm going to get is this, and actually, this is going to work out nicely. I'm just going to erase this H here. We're drawing the condensation mechanism, losing water, and now we can just reposition this guy. We've just lost our water and just made our beta-1-4 linkage, but we're going to have an extra H. We haven't lost our water completely yet; what we actually have lost is a water, and now we're about to lose an H to regenerate the acid. And now, I'm going to get my product, which would be the cellobiose. Why do you think this would lead to a really poor yield of cellobiose? It's due to a mixture of linkages; you'd get a mixture of alpha and beta, which means that if you were just to link at the 1-4 position, that would be a mixture of maltose and cellulose. But then, you'd also get a mixture of positions, right now we just got a 1-4, but it could have also been a 1-2 linkage, or a 1-3 linkage, or a 1-6 linkage. We drew this to be the beta-1-4 because I was asking for cellobiose, but this is very, very unlikely if I were to just react this in acid. All of these different permutations would happen, which would mean that I would have almost no cellobiose and I would just have a big conglomeration of disaccharides and trisaccharides, and they would all just be kind of like intermingling, and it would just not be a great synthesis. The proper synthesis of disaccharides in the lab requires the use of protecting groups and very special reagents to ensure everything goes where you want. In the human body, specific enzymes make sure that you make exactly the right disaccharides. But if you're in the lab, you can't just throw this with some acid and expect to get cellulose; you're going to get a mixture of everything.

Here’s what students ask on this topic:

What is a disaccharide and how is it formed?

A disaccharide is a carbohydrate composed of two monosaccharides linked by an O-glycosidic bond. This bond forms through a condensation reaction, which involves the joining of two sugar molecules with the release of a water molecule. The reaction results in the formation of an acetal linkage between the monosaccharides. The type of glycosidic linkage, whether alpha or beta, determines the specific properties and name of the disaccharide. For example, cellobios is formed from two D-glucose molecules with a beta-1,4 linkage.

What is the difference between alpha and beta linkages in disaccharides?

The difference between alpha and beta linkages in disaccharides lies in the orientation of the glycosidic bond relative to the anomeric carbon. In an alpha linkage, the oxygen atom in the glycosidic bond is on the opposite side (trans) of the ring compared to the CH₂OH group. In a beta linkage, the oxygen atom is on the same side (cis) as the CH₂OH group. This orientation affects the disaccharide's properties and its biological functions.

What is a condensation reaction in the context of disaccharide formation?

A condensation reaction in the context of disaccharide formation is a chemical process where two monosaccharides combine to form a disaccharide, releasing a water molecule in the process. This reaction involves the removal of a hydroxyl group (OH) from one monosaccharide and a hydrogen atom (H) from the other, resulting in the formation of an O-glycosidic bond. The reaction produces an acetal linkage, locking the sugars in a fixed position.

Why do disaccharides not undergo mutarotation?

Disaccharides do not undergo mutarotation because their structure is fixed once the O-glycosidic bond is formed. In monosaccharides, mutarotation involves the interconversion between alpha and beta anomers through the opening and closing of the ring structure. However, in disaccharides, the glycosidic bond locks the anomeric carbon in a specific configuration, preventing the ring from opening and thus stopping any further interconversion between anomers.

What are some common examples of disaccharides and their linkages?

Some common examples of disaccharides include sucrose, lactose, and maltose. Sucrose is composed of glucose and fructose with an alpha-1,2 linkage. Lactose consists of glucose and galactose with a beta-1,4 linkage. Maltose is made up of two glucose molecules with an alpha-1,4 linkage. Each of these disaccharides has unique properties and functions based on the specific monosaccharides involved and the type of glycosidic linkage.