Question

Draw the chemical structures of glucose and fructose in the open-chain and closed-ring conformations. Which of these simple sugars is an aldose and which is a ketose?

Short answer

Answer: The open-chain structure of glucose is an aldehyde with hydroxyl groups at the other carbons, while the open-chain structure of fructose is a ketone with hydroxyl groups at the other carbons. The closed-ring structure of glucose is a six-membered pyranose ring, while the closed-ring structure of fructose is a five-membered furanose ring. Glucose is an aldose due to its aldehyde group, and fructose is a ketose due to its ketone group.

Step-by-step solution

Draw open-chain glucose structure

First, we need to draw the open-chain structure of glucose. Glucose is a 6-carbon sugar with the molecular formula C6H12O6. Its structure contains an aldehyde group at the top carbon and hydroxyl groups (-OH) at the other carbons. H H H H H | | | | | O= C - C - C - C - C - C - H | | | | | H OH OH OH OH

Draw open-chain fructose structure

Similarly, we need to draw the open-chain structure of fructose. Fructose is also a 6-carbon sugar with the molecular formula C6H12O6. However, its structure has a ketone group at the 2nd carbon position and hydroxyl groups (-OH) at the other carbons. H H H H H | | | | | H - C - C = O C - C - C - H | | | OH OH OH

Identify aldose and ketose

Now, we can determine which sugar is an aldose and which is a ketose based on their functional groups. Glucose has an aldehyde group (O=CH), so it is an aldose. Fructose has a ketone group (C=O), which makes it a ketose.

Draw closed-ring glucose structure

To draw the closed-ring structure of glucose, we need to create a link between the first and fifth carbons. In doing this, a new chiral carbon is created with the sixth carbon attached to the fifth carbon. The result is a six-membered ring, known as a pyranose structure. O | HO - C - H | \ C ----- C - H / | | H - C H OH | | OH OH

Draw closed-ring fructose structure

Lastly, we will draw the closed-ring structure of fructose. This entails creating a link between the first and fourth carbons. A new chiral carbon is formed with the sixth carbon attached to the fifth carbon. The resulting structure is a five-membered ring, called a furanose structure. O | H - C - C - H | / C --- C - H | OH

Key concepts

Glucose

Glucose is one of the most essential carbohydrates, commonly referred to as a simple sugar or monosaccharide. It is a primary source of energy for living organisms and is found in the blood of animals as blood sugar. Its molecular formula is \( C_6H_{12}O_6 \). In its open-chain form, glucose has a linear structure with an aldehyde group at one end, making it an aldose sugar. This aldehyde group is located on the first carbon of the chain.

When glucose cyclizes, it forms a six-membered ring known as a pyranose structure, which is more stable than its open-chain counterpart. In this closed-ring conformation, glucose predominantly exists in two stereoisomers: alpha-glucose and beta-glucose. These isomers differ at the first carbon—known as the anomeric carbon—where the hydroxyl group can position itself either below (alpha) or above (beta) the plane of the ring.

Fructose

Fructose, often found in fruits, honey, and root vegetables, is another common monosaccharide with the same chemical formula as glucose \( C_6H_{12}O_6 \). However, its structure differs because it contains a ketone group on the second carbon, classifying it as a ketose sugar. The presence of this ketone group in its open-chain form grants fructose distinct chemical behaviors and reactions compared to glucose.

When fructose forms a closed-ring conformation, it results in a five-membered ring called a furanose structure. Similar to glucose, fructose can form different isomers, with the most common ones being alpha-fructose and beta-fructose. In solutions, fructose is mostly found in the ring form due to its increased stability, and this form contributes to the sweet taste of fructose.

Aldose and Ketose

The classification into aldose and ketose is based on the presence of functional groups in a carbohydrate's structure. An aldose contains an aldehyde group \( -CHO \) at the end of the carbon chain, while a ketose features a ketone group \( C=O \) typically on the second carbon atom.

This structural difference is crucial because it affects the sugar's reactivity, its physical properties, and the way each sugar forms cyclic structures. As we've discussed, glucose is an example of an aldose sugar due to its aldehyde group, and fructose is a ketose sugar due to its ketone group. Understanding these categories is essential for students studying biochemistry, as it lay the foundation for more complex topics like carbohydrate metabolism and glycoside formation.

Open-chain and Closed-ring Conformations

Carbohydrates, such as sugars, exist in equilibrium between open-chain and closed-ring conformations. The open-chain form is the straight-chain structure of the sugar, while the closed-ring form is a ring-shaped structure resulting from a reaction between the carbonyl group (aldehyde or ketone) and a hydroxyl group on the same molecule.

During cyclization, the carbonyl carbon becomes a new chiral center called the anomeric carbon, leading to the formation of two different anomers (alpha and beta). The cyclization process is reversible and in aqueous solutions, monosaccharides like glucose and fructose are predominantly found in their cyclic forms due to the stability of these forms in the watery environment of living cells.