Question

The term "epimer" is used to describe diastereomers that differ in the configuration about a single carbon atom. Which pairs of the eight possible aldopentoses are epimers?

Short answer

The pairs of epimers among the eight possible aldopentoses are (D-arabinose, D-ribose), (D-xylose, D-lyxose), (D-allose, D-glucose), and (D-altrose, D-mannose).

Step-by-step solution

Determine the Structure of Aldopentoses

Each of the eight aldopentoses can exist in D- and L- forms but here you only need to consider D-forms. \ - D-arabinose: HOCH2(CHOH)3CHO \ - D-xylose: HOCH2(CHOH)2CHOHCHO \ - D-lyxose: HOCH2CHOH(CHOH)2CHO \ - D-ribose: HOCH2CHOH(CHOH)CHOHCHO \ - D-allose: HOCH2CHOHCHOH(CHOH)2CHO \ - D-altrose: HOCH2CHOHCHOHCHOHCHO \ - D-glucose: HOCH2CHOHCHOH(CHOH)CHOHCHO \ - D-mannose: HOCH2CHOHCHOHCHOH(CHOH)2CHO

Identify Epimers

Now check the configurations of each aldopentose to see which pairs differ in the configuration of only one carbon atom. After analyzing each pair, the pairs of epimers are (D-arabinose, D-ribose), (D-xylose, D-lyxose), (D-allose, D-glucose), and (D-altrose, D-mannose)

Checking Understanding

Check your understanding by making sure that each pair of identified aldopentoses indeed differ only in the configuration of a single stereogenic carbon.

Key concepts

Aldopentoses

Aldopentoses are a group of monosaccharides composed of five carbon atoms, including an aldehyde group at the first carbon. These sugars play a vital role in the structure and function of essential biomolecules, such as nucleic acids. In nature, they typically appear in both D- and L-forms, depending on the orientation of their hydroxyl groups relative to the aldehyde group. D-forms are more common in nature and widely studied in biochemistry.

Some common aldopentoses include:

• D-arabinose
• D-xylose
• D-ribose
• D-lyxose

These sugars differ in the arrangement of hydroxyl groups on their carbon chain, resulting in diverse properties and biological roles. Aldopentoses are critical in energy production, metabolic pathways, and serve as building blocks for larger, complex carbohydrates. Understanding their structure is fundamental to grasping how they interact within biological systems.

Stereochemistry

Stereochemistry is the study of how molecules differ in their spatial arrangement, even when they share the same molecular formula. It's crucial when exploring carbohydrates like aldopentoses because subtle differences in orientation can lead to significant changes in function and interaction within biological systems. In the context of aldopentoses, stereochemistry is observed in the arrangement of hydroxyl groups around the asymmetric carbon atoms.

The concept of chirality is central to stereochemistry. Chiral molecules, such as aldopentoses, have non-superimposable mirror images, referred to as enantiomers. This property is important in biological contexts, where different enantiomers can have different biological activities. The distinct spatial orientation of molecules also leads to the formation of different diastereomers, providing a fascinating depth to carbohydrate chemistry. By understanding the stereochemical nature of aldopentoses, we gain insight into the diversity and functionality of sugars in living organisms.

Diastereomers

Diastereomers are stereoisomers that are not mirror images of each other. Unlike enantiomers, which differ at all stereocenters, diastereomers differ at one or more, but not all, stereocenters. This makes diastereomers quite intriguing, particularly in the study of carbohydrates like aldopentoses. Among these, epimers are a specific type where the difference occurs at only a single carbon atom.

For example, in aldopentoses, pairs like D-arabinose and D-ribose are epimers because they only differ in the configuration around one carbon atom. Similarly, D-xylose and D-lyxose form another pair of epimers. These differences might seem minor but can drastically alter the biological characteristics of the sugars. Diastereomers are imperative in carbohydrate chemistry for understanding structural diversity, informing both their functional role in metabolism and their broader application in synthetic carbohydrate chemistry.