Question

(+) Sucrose is (a) \(\alpha\) -D-glucopyranosyl- \(\beta\) -D-fructopyranoside (b) \(\alpha\) -D-glucopyranosyl- \(\beta\) -D-fructofuranoside (c) \(\alpha\) -D-glucopyranosyl- \(\beta\) -D-fructofuranoside (d) \(\alpha\) -D-glucopyranosyl- \(\alpha\) -D-fructopyranoside

Short answer

Answer: The correct structure of sucrose is α-D-glucopyranosyl-β-D-fructofuranoside.

Step-by-step solution

Recognize the glucose and fructose forms in sucrose

Sucrose is formed by the combination of one glucose molecule and one fructose molecule. Glucose is found in a cyclic pyranose form, and fructose is generally found in a cyclic furanose form. Knowing these structures, we can narrow down our options by eliminating those that don't match these forms.

Eliminate incorrect options

We can eliminate options where glucose is not in pyranose form or fructose is not in furanose form. Therefore, options (a) and (d) can be eliminated. Now, we are left with two options: (b) \(\alpha\) -D-glucopyranosyl- \(\beta\) -D-fructofuranoside (c) \(\alpha\) -D-glucopyranosyl- \(\beta\) -D-fructofuranoside

Identify the glycosidic bond in sucrose

Sucrose is formed by linking the \(\alpha\)-anomer of glucose (at the C1 position) and the \(\beta\)-anomer of fructose (at the C2 position). This bond formation results in the formation of an \(\alpha\) -D-glucopyranosyl- \(\beta\) -D-fructofuranoside structure.

Choose the correct option

By recognizing the correct anomeric forms of glucose and fructose molecules and the glycosidic bond in sucrose, we can now identify the correct option for the structure of sucrose. The correct option is (b) \(\alpha\) -D-glucopyranosyl- \(\beta\) -D-fructofuranoside.

Key concepts

Glycosidic Bond

Understanding the intricate nature of molecular connections in carbohydrates is essential for grasping how these biological substances function. One such vital linkage is the glycosidic bond, which is a type of covalent bond that joins a carbohydrate (sugar) molecule to another group, which can be another sugar or a different type of molecule altogether.

Glycosidic bonds form through a dehydration reaction, where a water molecule is released as the bond is established. In the specific case of sucrose, a glycosidic bond forms between the anomeric carbon atom of glucose and the anomeric carbon of fructose. This bond is the defining feature that links individual sugar units into more complex carbohydrates, which can be broken down through hydrolysis. Such reactions are not only pivotal in carbohydrate chemistry but also play a significant role in various biological processes, including energy storage and retrieval.

Anomers of Glucose and Fructose

Anomers are forms of monosaccharides that differ in the configuration around the anomeric carbon. The anomeric carbon is typically the carbon derived from the carbonyl group (carbon double-bonded to oxygen) during the formation of a cyclic saccharide. In glucose and fructose, these are C1 and C2 respectively.

On their own, glucose typically forms a six-membered ring known as pyranose, and fructose forms a five-membered ring referred to as furanose. Different anomeric forms, namely alpha (α) and beta (β), are determined by the relative position of the hydroxyl group (-OH) attached to the anomeric carbon compared to the rest of the molecule. For instance, if the hydroxyl group is on the opposite side of the ring as the CH2OH group, it's considered the alpha anomer. If it's on the same side, it's the beta anomer. These subtle changes can have significant implications on the properties and reactivity of the resulting sugars.

Alpha-D-Glucopyranosyl-Beta-D-Fructofuranoside

Diving deeper into the structure of sucrose, we encounter alpha-D-glucopyranosyl-beta-D-fructofuranoside. This term describes the specific way in which glucose and fructose are linked together to form sucrose—glucose presents as alpha-D-glucopyranose, and fructose presents as beta-D-fructofuranose.

It's the combination of these two specific anomers that gives sucrose its unique properties. The glucose part is in the form of a pyranose ring, resembling the structure of pyran, a six-membered ring with five carbon atoms and one oxygen atom. The fructose part, on the other hand, forms a five-membered furanose ring composition, likened to furan. The nomenclature informs chemists about both the type of sugar present and the specific orientation of atoms in the molecule, which is critical for understanding its 3D structure and reactivity.

Carbohydrate Chemistry

Carbohydrate chemistry is a fascinating and broad field that studies the various aspects of carbohydrates, including their structures, functions, and how they interact with other biomolecules. Carbohydrates themselves are crucial biological molecules that serve as energy sources and structural components in living organisms.

Within this field, we explore the characteristics of simple sugars (monosacarides), the formation of complex sugars (disaccharides, oligosaccharides, and polysaccharides), and the reaction mechanisms, such as the formation and breaking of glycosidic bonds. From a biochemical perspective, the study of carbohydrate chemistry provides insights into processes like energy metabolism, cell-cell recognition, and the molecular basis of certain diseases. Understanding the structural and functional nuances of carbohydrates like sucrose enables students to appreciate both the diversity and specificity of these essential biomolecules.