In which disaccharide glucosidic linkage is formed between anomeric \(-\mathrm{OH}\) of glucose and anomeric \- OH of fructose? (a) Lactose (b) Maltose (c) Cellobiose (d) Sucrose

Monosaccharides are the simplest form of carbohydrates, often referred to as the building blocks of more complex sugars. They consist of a single sugar molecule with the general formula \( C_nH_{2n}O_n \), where \( n \) can be any number of three or more. The most common monosaccharides are glucose, fructose, and galactose, and they can exist in both linear and ring forms. In aqueous solutions, they predominantly exist as cyclic structures, which are the result of a reaction between a carbonyl group and a hydroxyl group in the same molecule, creating a hemiacetal or hemiketal.

Understanding the structure of monosaccharides is crucial for studying disaccharides, as they are the components from which disaccharides are formed through glycosidic linkages. These single sugar units determine many of the properties of the larger sugar molecule, including its reactivity and the way it interacts with other biological molecules.

Sucrose is a disaccharide that is commonly known as table sugar. Its structure is a combination of two monosaccharides: glucose and fructose, linked together by a glycosidic bond. In sucrose, glucose and fructose are in their ring forms, known as furanose for fructose and pyranose for glucose, and they are linked together in a specific manner.

To understand the connection, let's look at the glycosidic linkage. In sucrose, this bond is between the anomeric carbon (C-1) of glucose and the anomeric carbon (C-2) of fructose, both of which originally had a hydroxyl group \( -OH \) prior to bond formation. When these two hydroxyl groups react, they release a molecule of water \( H_2O \) and form a covalent bond, making sucrose a non-reducing sugar because it no longer has a free anomeric carbon. An easy way to visualize the structure is to imagine the two rings sitting side by side with a bridge between them, where the bridge represents the glycosidic bond.

Anomeric carbon plays a key role in sugar chemistry. It is the carbon atom that was originally part of the carbonyl group \( C=O \) before the formation of the ring structure in a monosaccharide. When the ring forms, the carbonyl carbon becomes a new chiral center, dubbed the anomeric carbon, and it can take on one of two configurations: alpha (α) or beta (β). The position of the hydroxyl group \( -OH \) attached to the anomeric carbon determines these configurations; if the \( -OH \) group is on the opposite side of the ring as the CH2OH moiety, it is alpha, if it's on the same side, it is beta.

The property of being able to exist in two forms is called 'anomerism,' and the two forms, α and β, are called 'anomers.' The significance of the anomeric carbon in disaccharides, like the exercise example involving sucrose, is that it's the carbon through which the glycosidic linkage forms, directly influencing the structure and properties of the resulting disaccharide. Understanding the concept of the anomeric carbon is essential when studying the structure of sugars and their reactivity.