Osazone test

This test is used for the identification of sugars. It involves the reaction of monosaccharide with phenyl hydrazine, a crystalline compound. All reducing sugars form osazones with excess of phenyl hydrazine when kept at boiling temperature. Each sugar has a characteristic crystal form of osazones.

Which “one” of the following shapes corresponds with the shape of sucrosazone crystals?

A. Needle shaped

B. Petal shaped

C. Rhombic plates

D. Powder puff shaped

E. None of the above.

Details

The correct answer is E- Sucrose is a non reducing sugar, it does not form osazone crystals. Upon hydrolysis, the products glucose and fructose form needle shaped crystals.

Basic concept

Osazone test

The reaction involved can be represented as follows-

 

Phenyl hydrazine reaction

Figure-1- Reaction showing formation of osazones

Three molecules of phenylhydrazine are required, the reaction takes place at first two carbon atoms. The upper equation shows the general form of the osazone reaction, which affects an alpha-carbon oxidation with formation of a bis-phenylhydrazone, known as an osazone.

D-fructose and D-mannose give the same osazone as D-glucose. The difference in these sugars present on the first and second carbon atoms are masked when osazone crystals are formed. Hence these three sugars form similar needle-shaped crystals arranged like sheaves of corn or a broom (figure-2).  It is seldom used for identification these days . HPLC or mass spectrometry is used for the identification of sugars present in the biological fluids.

Needle shaped crystals

Figure-2- Needle shaped crystals of Glucose, Mannose and Fructose

Sucrose has no free reactive group because the anomeric carbons of both monosaccharides units are involved in the glycosidic bond, the configuration of this glycosidic linkage is α-for glucose and β-for fructose (Figure-3). Hence , sucrose neither shows reducing nor mutarotation characters.

Sucrose

Figure-3- Structure of sucrose

As regards other options

A. Needle shaped crystals are formed by Glucose, fructose and Mannose

B. Petal shaped-crystals are formed by Maltose

C. Rhombic plate shaped crystals are formed by Galactose

D. Powder puff shaped-crystals are formed by Lactose

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5 Responses

  1. I have got my doubt from this description.
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  4. Anusha Durai says:

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