Cyclic Structures of Monosaccharides: Study with Video Lessons, Practice Problems & Examples

Monosaccharides, such as d-glucose, exist as cyclic hemiacetals in aqueous solutions through cyclization, where the penultimate alcohol reacts with the aldehyde group. This process creates two anomers: alpha and beta, distinguished by the orientation of the hydroxyl group at the anomeric carbon (carbon 1). In the alpha form, the hydroxyl group is down, while in the beta form, it is up. Understanding these configurations is crucial for studying carbohydrates and their biochemical roles, including their participation in metabolic pathways like glycolysis and glycogenesis.

Cyclic Structures of Monosaccharides Concept 1

Here we're going to say that monosaccharides exist as cyclic hemiacetals in aqueous solutions. Now cyclization takes place when the penultimate alcohol reacts with the C1 aldehyde group. If we take a look here in the center, we have the acyclic monosaccharide in the form of D-glucose. And we're talking about the penultimate alcohol, so we're talking about this alcohol here. This alcohol is what's going to help to make this within our structure. Now from this acyclic form, we're able to create two possibilities. One where the OH is pointed down when it comes to this hemiacetyl carbon, and one where it's pointing up. But how do we know what they are called? Well, this is where the term anomers comes into play. Anomers, these are just epimers produced by cyclization of monosaccharides. Remember, an epimer means that we have the same configuration at all chiral centers except for one place. If we were to look, all the chiral centers are the same everywhere except for carbon number 1, because that OH could either point down or point up. That gives us 2 epimers or anomers of each other. Here we're gonna say alpha anomer. This is where the OH group and the CH₂OH group are on opposite sides of each other. If we can see, the OH group is pointed down here, but the CH₂OH group is pointed up. They're opposites of each other, so this would be alpha D-glucose. And then beta anomer, this is where our anomeric OH and C6 CH₂OH are on the same side. This will be beta. Now, remember carbon number 1, we can call it the hemiacetyl carbon, but we can also call it the anomeric carbon because it can give us 2 possibilities. The OH could orient itself down, giving us alpha, or could orient itself up, giving us beta. These would be epimers or anomers of each other. Alright. So just remember, we're going from the noncyclic form of glucose, and once we put it in an aqueous environment, it's gonna coil up to form either this ring or this ring as its dominant forms. So keep that in mind when we're talking about different types of cyclic hemiacetals that originate from an acyclic monosaccharide.

Cyclic Structures of Monosaccharides Example 1

Here it says draw on Haworth projection for beta-D-galactose. Alright. So step 1 is we're going to number the Fischer projection. So 1, 2, 3, 4, 5, and 6. And rotate it clockwise to turn it on its side. So now it's 123456. Notice that we're starting out with D-galactose. We don't get beta until it's in its cyclic form. Step 2 now is we're going to curl the CH₂OH group clockwise, keeping the carbonyl group in the far right corner. So here we're gonna kind of like bend this structure, it's linear right now, we're gonna bend it a little bit, curve upon itself, so that my CH₂OH group and my carbonyl group are in similar positions, similar area with each other. Now, this is gonna be a little bit tricky. We're going to rotate C₅, so carbon number 5 here. So, CH₂OH faces up, bringing OH group close to the carbonyl group. Alright. So basically, we're gonna have this OH and this CH₂OH moving. So this COH is gonna move to where CH₂OH is, and then C₅H₂OH is gonna move here. So that we get now, CH₂OH up here, and then the OH rotated over here. And here, we're gonna close the ring to form the cyclic hemiacetal and assign alpha or beta to the anomeric OH group. Remember, the anomeric group is carbon number 1. It is the carbon that was the carbonyl group here. Alright. So basically, this OH and this Carbon are gonna form a bond with each other, this bond here. Oxygen ideally wants to only make 2 bonds, so it's gonna lose its H. And we're gonna say that this double bonded O is no longer gonna be double bonded because carbon can't make 5 bonds. It's gonna transform into an OH group. We said here beta, which means that the OH group has to be on the same side as a CH₂OH group. So OH would be up here, oriented up here. And this H is still around and it'd have to be down here. So this here will represent our beta-D-galactose structure.

Draw a Haworth projection for α-D-altrose.

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D-ribose is an aldopentose sugar that is found in the DNA. It commonly exists as a five-membered β anomer. Draw D‑ribose in its cyclic hemiacetal form.

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Here’s what students ask on this topic:

What are the cyclic structures of monosaccharides?

Monosaccharides, such as d-glucose, can form cyclic structures known as hemiacetals in aqueous solutions. This cyclization occurs when the penultimate alcohol group reacts with the aldehyde group, forming a ring structure. The most common cyclic forms are five-membered (furanose) and six-membered (pyranose) rings. For example, d-glucose typically forms a six-membered ring. The cyclization creates two anomers: alpha and beta, which differ in the orientation of the hydroxyl group at the anomeric carbon (carbon 1). In the alpha anomer, the hydroxyl group is down, while in the beta anomer, it is up.

How do alpha and beta anomers of glucose differ?

Alpha and beta anomers of glucose differ in the orientation of the hydroxyl group at the anomeric carbon (carbon 1). In the alpha anomer, the hydroxyl group is positioned down relative to the plane of the ring, while in the beta anomer, it is positioned up. This difference in orientation occurs during the cyclization of the linear form of glucose into its cyclic form. These anomers are a type of epimer, meaning they have the same configuration at all chiral centers except for the anomeric carbon.

What is the significance of the anomeric carbon in monosaccharides?

The anomeric carbon in monosaccharides is significant because it is the carbon that becomes a new chiral center during the cyclization process. In glucose, this is carbon 1. The orientation of the hydroxyl group attached to the anomeric carbon determines whether the sugar is in the alpha or beta form. This distinction is crucial for the sugar's biochemical properties and its role in metabolic pathways. The anomeric carbon is also the site of glycosidic bond formation when monosaccharides link to form disaccharides and polysaccharides.

How does cyclization of monosaccharides occur?

Cyclization of monosaccharides occurs when the penultimate alcohol group (the hydroxyl group on the second-to-last carbon) reacts with the aldehyde or ketone group. In the case of glucose, the hydroxyl group on carbon 5 reacts with the aldehyde group on carbon 1, forming a hemiacetal. This reaction creates a ring structure, typically a six-membered ring (pyranose) for glucose. The cyclization can result in two different anomers, alpha and beta, depending on the orientation of the hydroxyl group at the anomeric carbon.

What role do cyclic structures of monosaccharides play in metabolism?

Cyclic structures of monosaccharides play a crucial role in metabolism. For instance, glucose in its cyclic form is a key substrate in glycolysis, where it is broken down to produce energy. The cyclic form also participates in glycogenesis, where glucose molecules are polymerized to form glycogen for energy storage. The specific anomeric form (alpha or beta) can influence the enzyme interactions and the efficiency of these metabolic pathways. Understanding these cyclic structures is essential for studying carbohydrate metabolism and energy production in cells.