Question

D-Glucose and D-Mannose are: (1) Anomers (2) Enantiomers (3) Geometrical Isomers (4) Epimers

Short answer

(4) Epimers

Step-by-step solution

Understand the Definitions

Before identifying the relationship between D-Glucose and D-Mannose, let's clearly understand each option given: 1. Anomers: Isomers that differ at a new asymmetric carbon atom formed on ring closure.2. Enantiomers: Isomers that are mirror images of each other.3. Geometrical Isomers: Isomers that differ in the spatial arrangement of groups around a double bond or ring system.4. Epimers: Isomers that differ at one specific carbon atom.

Compare Structural Differences

Identify the structural difference between D-Glucose and D-Mannose. Both are aldohexoses, but they differ at a specific carbon. D-Glucose and D-Mannose have different configurations at carbon 2.

Identify the Correct Relationship

Based on the definitions and structural comparison, D-Glucose and D-Mannose differ only at the carbon 2 position. This means they are epimers.

Confirm the Answer

Since they differ at exactly one carbon atom (carbon 2), the correct answer is (4) Epimers.

Key concepts

D-Glucose

D-Glucose is an aldohexose, meaning it is a six-carbon sugar with an aldehyde group. This sugar is one of the most important carbohydrates in biology. It serves as an essential source of energy. D-Glucose is found naturally in fruits, honey, and various plant juices. The molecule has several chirality centers (carbon atoms with four different groups attached), which contribute to its properties and how it reacts to form different isomers.
In its linear form, it has a carbonyl group at C1 and hydroxyl groups on the remaining carbons. When D-Glucose forms a ring, the carbonyl group reacts with a hydroxyl group to form either a six-membered ring (pyranose form) or a five-membered ring (furanose form). The ring formation introduces another isomerism known as anomerism, which leads to the formation of alpha (α) and beta (β) anomers.

D-Mannose

D-Mannose is another important aldohexose, similar to D-Glucose. The primary difference between D-Mannose and D-Glucose is the configuration of the hydroxyl group on the second carbon (C2). This slight structural variation can have significant effects on their biochemical roles and properties.
Like D-Glucose, D-Mannose can also exist in both a linear and cyclic form. The cyclic form can be in either a pyranose or a furanose structure. D-Mannose plays an important role in glycoprotein synthesis and cell recognition processes. It is less common in the human diet than glucose but still vital in specific biological functions.
Understanding this structural difference helps explain why they are classified as epimers, as they only differ at one specific carbon atom, which in this case is C2.

Epimers

Epimers are a specific type of stereoisomer found in carbohydrates. They differ from each other at only one specific carbon atom of the molecule. This is vital in biochemistry because even small changes can lead to significant differences in how molecules interact biologically.
For example:

• D-Glucose and D-Mannose are epimers at carbon 2 (C2). This single difference makes them exhibit different properties.
• The epimeric difference influences how these sugars react enzymatically and how they are utilized in various metabolic pathways.
• Epimers can have significant implications in medical and nutritional sciences.

Understanding isomerism, including epimers, helps in comprehending how subtle differences can change the function and behavior of molecules fundamentally.

Carbohydrate Isomerism

Carbohydrate isomerism refers to the phenomenon where compounds with the same molecular formula have different structural arrangements. These differences can lead to various types of isomers:

• **Anomers**: Are isomers that differ at the newly formed asymmetric carbon atom after the ring structure is formed.
• **Enantiomers**: Are mirror image isomers. They are non-superimposable, meaning that no matter how you rotate them, they won't match.
• **Geometrical Isomers**: Also known as cis-trans isomers, these differ in the spatial arrangement due to the presence of a double bond or a ring structure.
• **Epimers**: As mentioned earlier, these differ at only one specific carbon atom.

The study of carbohydrate isomerism is crucial in fields like medicinal chemistry and biochemistry because these isomers can have vastly different effects in biological systems.
Correctly identifying the type of isomerism can help in understanding metabolic pathways, drug design, and diagnosing metabolic disorders.