Know all about the concept of Biomolecules | Chemistry

Table of Contents

1. Carbohydrate

1.1 D-(+) – Glucose

1.2 Chemical properties

1.3 Reaction of carbohydrates with phenylhydrazine

1.4 Disaccharides

1.5 Reducing and non-reducing sugars

1.6 Amino Acids

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1. Carbohydrate

1.1 D-(+) – Glucose

Glucose is prepared by the hydrolysis of sucrose, while commercially it is prepared by the hydrolysis of starch.

• • Glucose is a white crystalline substance
  • Soluble in water, sparingly soluble in alcohol.
  • It is dextrorotatory, hence also called dextrose.
  • It shows mutarotation (change in specific rotation in aqueous solution of the compound).

1.2 Chemical properties

(i)

(ii)

(iii)

(iv)

(v)

1.3 Reaction of carbohydrates with phenylhydrazine

Both of the phenylhydrazine residues of osazone can be removed to form dicarbonyl compounds, known as osones.

Osazones are yellow coloured, crystalline compounds. osazone formation involves only the first two carbon atoms, i.e. CHOCHOH- in aldoses and  in ketoses, of a compound without affecting the configuration of the rest of the molecule. Thus hexoses having similar configuration on  and  will form same osazone, e.g. glucose, mannose and fructose.

Tests of glucose

(i) Molisch’s test : This is a general test for carbohydrates. A drop or two of alcoholic solution of a-naphthol is added to 2 mL of glucose solution. concentrated is added carefully along the sides of the test tube. The formation of a violet ring, at the junction of two liquids confirms the presence of a carbohydrate.

(ii) Silver mirror test : A mixture of glucose and ammonical silver nitrate is warmed in a test tube. Appearance of silver mirror on the inner walls confirms glucose.

(iii) Fehling’s solution test : Glucose is warmed with Fehling’s solution. A red precipitate of cuprous oxide is formed.

(iv) Osazone formation : Glucose on heating with excess of phenylhydrazine gives a yellow crystalline compound.

Structure of Glucose

(i) Glucose exists in two stereoisomeric forms (a and b). a -glucose with specific rotation +110⁰ , whereas b-glucose with specific rotation +19.7⁰

(ii) A aqueous solution of glucose shows mutarotation, i.e., its specific rotation gradually decreases from +110⁰ to 52.5⁰ in case of a-glucose and  increases from 19.7⁰  to 52.5⁰ in case of b-glucose.

The two forms of D-glucose are also shown by Haworth projection formula which are given below :

1.4 Disaccharides

The disaccharides yield on hydrolysis two monosaccharides.

Sucrose or cane-sugar (C₁₂H₂₂O₁₁) :         It is dextrorotatory but does not show mutarotaton. It is a non-reducing sugar as it does not reduce Tollen’s or Fehling’s reagent.

It is composed of a-D-glucopyranose unit and a b-D-fructofuranose unit.

These units are joined by a-b-glycosidic linkage between C-1 of the glucose unit and C-2 of the fructose unit.

Inversion of cane-sugar 

The hydrolysis of sucrose by boiling with a mineral acid or by enzyme invertase or sucrose, produces a mixture of equal molecules of D(+) glucose and D(-) fructose.

Thus, in the process of hydrolysis of sucrose, is termed as inversion  of sugar and the hydrolysed mixture having equal molar quantities of D(+) glucose and D(-) fructose is called invert sugar. The enzyme that brings the inversion is named as invertase.

1.5 Reducing and non-reducing sugars

1.6 Amino Acids

Amino acids :  An amino acid is a bifunctional organic molecule that contains both a carboxyl group, -COOH, as well as an amine group, .

In an a – amino acid, the amine group is located on the carbon atom adjacent to the carboxyl group (the a -carbon atom). The general structure of the a -amino acids is represented as :

Methods of Preparation of a–Amino Acids

(i) Amination of a –halo acids :

(ii) Gabriel phthalimide synthesis :

(iii) Knoop synthesis :

(iv) Strecker synthesis :

Properties :

(i) Except glycine, all the a -amino acids are optically active.

(ii) Zwitter ion and isoelectric point : the – NH₂ group is basic and –COOH group is acidic, in neutral solution, it exists in an internal ionic form called a Zwitter ion, also known as dipolar ion.

The Zwitter ion is dipolar, charged but overall electrically neutral and contains both a positive and negative charge.

When an ionised form of amino acid is placed in an electric field, it will migrate towards the opposite electrode. Depending on the pH of the medium, following three things may happen :

(i) In acidic solution (low pH), the positive ion moves towards cathode [exist as cation, structure (II)].

(ii) In basic solution (high pH), the negative ion moves towards anode [exist as anion, structure (III)].

(iii) The Zwitter ion does not towards any of the electrodes [neutral dipolar ion, structure (I)].

The intermediate pH at which the amino acid shows no tendency to migrate towards any of the electrodes and exists the equilibrium when placed in an electric field is known as isoelectric point.