Question

Which of the following carbohydrates undergo mutarotation? (a) Maltose (b) Sucrose (c) Cellulose (d) Fructose

Short answer

a) Maltose b) Sucrose c) Cellulose d) Fructose Answer: a) Maltose and d) Fructose

Step-by-step solution

Understanding Maltose

Maltose is a disaccharide composed of two glucose units linked by an α(1→4) bond. Both its individual units, glucose, can exist in the open-chain form (hemiacetal and hemiketal) and the cyclic form (pyranose and furanose). Thus, maltose can undergo mutarotation when its glucose units interconvert. Therefore, maltose undergoes mutarotation.

Understanding Sucrose

Sucrose is a disaccharide composed of one glucose unit and one fructose unit linked by an α(1→2) bond. This bond is quite stable and does not allow its units to exist in their open-chain forms. This locked anomeric configuration prevents sucrose from undergoing mutarotation. Therefore, sucrose does not undergo mutarotation.

Understanding Cellulose

Cellulose is a polysaccharide composed of glucose units linked together by β(1→4) bonds. Due to its linear and stable structure, it prevents the glucose units from existing in their open-chain forms and exchanging between conformations. Thus, cellulose does not undergo mutarotation.

Understanding Fructose

Fructose is a monosaccharide that can exist in open-chain form (hemiacetal and hemiketal) or in cyclic form (pyranose and furanose). Interconversion between these forms leads to mutarotation. Therefore, fructose undergoes mutarotation.

Conclusion

Among the given carbohydrates, maltose and fructose can undergo mutarotation, while sucrose and cellulose do not. So the answer is (a) Maltose and (d) Fructose.

Key concepts

Carbohydrates

Carbohydrates are one of the essential biomolecules that play a crucial role in our daily diet. They are composed of carbon, hydrogen, and oxygen, usually with the general formula \( (CH_2O)_n \). Carbohydrates serve as an important source of energy for living organisms. These molecules exist in various forms, including monosaccharides, disaccharides, oligosaccharides, and polysaccharides.

• Monosaccharides: Simple sugars like glucose or fructose.
• Disaccharides: Composed of two monosaccharide units, such as sucrose or maltose.
• Polysaccharides: Long chains of monosaccharide units, like cellulose.

The process of carbohydrates transitioning between different structural forms is integral to their functionality in biological systems, especially in terms of energy availability and storage.

Disaccharides

Disaccharides are a type of carbohydrate consisting of two monosaccharide units linked together by a glycosidic bond. When these glycosidic bonds form, the individual sugars lose a molecule of water. This link can significantly influence the properties of the disaccharide.

• For example, Maltose consists of two glucose molecules linked by an α(1→4) bond. This bond allows maltose to participate in mutarotation because each glucose can adopt either an open-chain or cyclic form.
• Sucrose, on the other hand, is formed by an α(1→2) bond between glucose and fructose. This stable linkage does not favor any open-chain form, making sucrose unable to undergo mutarotation.

Understanding these differences is critical, as it affects how each sugar interacts in biological contexts.

Monosaccharides

Monosaccharides are the simplest form of carbohydrates and are often referred to as simple sugars. They cannot be hydrolyzed into simpler units. Common examples include glucose, fructose, and galactose. Monosaccharides have the general formula \( C_nH_{2n}O_n \). They can exist in multiple forms, depending on whether they are in a linear or ring structure.

• Open-chain form: The straight structure where the carbonyl group (either an aldehyde or ketone) is exposed.
• Cyclic form: A ring structure that forms when the carbonyl group reacts with a hydroxyl group along the sugar chain, creating a hemiacetal or hemiketal.

The ability to interconvert between these forms is a key characteristic of monosaccharides like fructose, which can undergo mutarotation, a natural phenomenon that involves a change in optical rotation as equilibrium between its forms is established.

Cyclic Forms

Cyclic forms of carbohydrates are crucial for their stability and reactivity. Carbohydrates often transition into these forms to reduce their reactivity and increase stability within cells.
This form arises when the carbonyl group (either aldehyde or ketone) reacts with an alcohol group within the same molecule, creating a ring structure.

• The most common cycle sizes are the five-membered furanose and the six-membered pyranose rings.
• For instance, glucose typically forms a pyranose ring, while fructose can form either a pyranose or furanose form.

These rings can open and close, allowing mutarotation. This property is involved in adjusting the positions of hydroxyl groups and changing the stereochemistry of the anomeric carbon, leading to different anomers (alpha or beta) that interconvert in solution.

Open-chain Forms

Open-chain forms of carbohydrates refer to their linear structure, which allows the aldehyde or ketone functional group to be free. This form is less common in solution for most sugars because it is more reactive and less stable compared to cyclic forms.
However, the open-chain form is crucial for chemical reactions, such as oxidation-reduction, and it plays a significant role in the mutarotation process.

• In the open-chain form, the carbonyl carbon is accessible, making it non-cyclic.
• This structure can be converted into a cyclic form when the aldehyde or ketone reacts with a hydroxyl group further along the chain, forming a hemiacetal or hemiketal linkage.

The ability of sugars like glucose and fructose to switch between open-chain and cyclic forms is essential to their role in biological processes, enabling them to undergo mutarotation and adapt to different structural requirements.