Question

The structural formula for the linear form of D-mannose is O=CC(O)C(O)C(O)C(O)CO (a) Is this molecule a sugar? (b) How many chiral carbons are present in the molecule? (c) Draw the structure of the sixmember-ring form of this molecule.

Short answer

(a) Yes, the molecule is a sugar. (b) There are 4 chiral carbons in the molecule. (c) Six-member-ring form of D-mannose: ``` O | HO-C1-C2 | \ C5-C6-HO / | OH OH | | O OH |\ C4-C3 | OH ```

Step-by-step solution

Understand the given structural formula

The linear form of D-mannose is given as: \(O=CC(O)C(O)C(O)C(O)CO\). We have a chain of carbons with an aldehyde functional group (O=), and each carbon except the last one has a hydroxyl group (OH) attached to it.

Determine if D-mannose is a sugar

A sugar molecule (also known as a saccharide) is an organic compound composed of carbon, hydrogen, and oxygen atoms, typically in a ratio of 1:2:1. They contain a carbonyl group (C=O) and several hydroxyl groups (OH). In the given structural formula, we have carbon, hydrogen, and oxygen atoms fulfilling the mentioned criteria with an aldehyde functional group and hydroxyl groups. Therefore, we can conclude that the given molecule is a sugar. Answer (a): Yes, the molecule is a sugar.

Calculate the number of chiral carbons

A chiral carbon is a carbon atom bonded to four different groups or atoms. To determine the number of chiral carbons in the D-mannose molecule, we need to analyze each carbon atom within the molecule. In the given structural formula, we can see that the first carbon with the aldehyde functional group and the last carbon with a single hydroxyl group only have three different groups attached, so they are not chiral. However, the other four carbons in the molecule are attached to four different groups (H, OH, the carbon to their right, and the carbon to their left). Therefore, they are considered chiral carbons. Answer (b): There are 4 chiral carbons in the molecule.

Draw the six-member-ring form of D-mannose

The six-member-ring form of a sugar molecule is called a pyranose. To form a pyranose ring, we must create a bond between the oxygen in the hydroxyl group on the second-last carbon atom and the first carbon atom containing the carbonyl group. The carbonyl group will convert into a hydroxyl group during this process. Here is the structure of the six-member-ring form of D-mannose: ``` O | HO-C1-C2 | \ C5-C6-HO / | OH OH | | O OH |\ C4-C3 | OH ``` Answer (c): See above for the drawn structure of the six-member-ring form of D-mannose.

Key concepts

Sugar Molecule

When we refer to a sugar molecule, we're talking about a crucial component of carbohydrates, which are essential sources of energy in biology. Sugars belong to a larger group called saccharides, and these are made up of carbon (C), hydrogen (H), and oxygen (O) atoms typically in a 1:2:1 ratio.

The linear form of D-mannose is presented as a chain of carbon atoms, each attached to hydrogen and oxygen to form hydroxyl groups (OH), with the exception of the first carbon that carries an aldehyde group (O=). This combination, along with the ring structure it can form, accounts for the general property of being sweet-tasting, soluble in water, and capable of undergoing fermentation. Sugars, including D-mannose, play an essential role not just in nutrition but also in cellular structure and function.

Chiral Carbons

When we explore the concept of chiral carbons, we dive into the three-dimensional world of molecular structures. A chiral carbon atom is one that is attached to four different atoms or groups, making it asymmetrical. This asymmetry is important because it means that the carbon and its attached groups can exist in two mirror-image forms, known as enantiomers.

Enantiomers are important in biochemistry because they can interact differently with other chiral molecules, such as enzymes or receptors. In the case of D-mannose, we find that within its linear form, four of the carbon atoms meet the criteria for chirality. They're each bonded to different groups: a hydrogen atom (H), a hydroxyl group (OH), and two other carbon atoms. These chiral carbons are central to the molecule's biological function and contribute to D-mannose's ability to interact with various biomolecules in specific manners.

Pyranose Form

Moving on to the pyranose form, this term relates to the ring-shaped structure that many sugar molecules can adopt. For D-mannose, the pyranose form is a six-membered ring comprising five carbon atoms and one oxygen atom.

This ring formation occurs when the hydroxyl group from one end of the molecule reacts with the aldehyde group at the other end, essentially bridging a loop with the oxygen atom joining two carbon atoms. The resulting cyclic structure is often more stable and predominates under physiological conditions. The pyranose form of sugars like D-mannose is critical for their biological activity, particularly when binding to other molecules or fitting into enzymes during metabolism. Learning to draw and recognize the pyranose form is key for students to understand how sugars function in living organisms.