Which one of the following statements is not correct?

A. D (-) fructose exists in furanose structure

B. D (+) glucose exists in pyranose structure

C. In sucrose the two monosaccharides are held together by peptide linkage

D. Maltose is a reducing sugar

E. Pentaacetate of glucose does not react with hydroxylamine

Choose the correct answer from the options given below:

C only

The correct answer is option 4. C only.

Let us explain why statement C is incorrect:

Sucrose is a disaccharide composed of two monosaccharides: glucose and fructose. The linkage between these two monosaccharides in sucrose is not a peptide linkage, which is characteristic of proteins, but rather a glycosidic linkage.

A glycosidic linkage is a type of covalent bond formed between the anomeric carbon atom of one monosaccharide and a hydroxyl group of another monosaccharide. In the case of sucrose, the glycosidic linkage is formed between the anomeric carbon of glucose and the hydroxyl group of fructose. Specifically, sucrose is formed by an α(1→2) glycosidic bond between the α-anomeric carbon of glucose and the hydroxyl group at carbon 2 of fructose.

This glycosidic bond is responsible for joining the two monosaccharides together to form the disaccharide sucrose.

Therefore, statement C is incorrect because it incorrectly describes the type of linkage present in sucrose. Instead of a peptide linkage, sucrose contains a glycosidic linkage.