Amino Acids

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Head,

Department of Chemistry

Gramin Mahavidyalaya

Vasantnagar(K.)

B. Sc. T.Y.

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Introduction

• Amino acids are bifunctional compounds containing both an

amino and a carboxyl group.

• These acids contain atleast one amino and one carboxyl group.
• The amino group could be linked to the carbon just next to

carboxyl group or with the gap of one or two carbons.

• They are called as α, β or γ acids respectively.
• α-amino acetic acid (2-amino ethanoic acid) NH₂ – CH₂– COOH
• β- amino propionic acid ( 3 – amino propionic acid)

NH₂ – CH₂– CH₂ – COOH

• γ – amino butyric acid (4 – amino butanoic acid)

NH₂ –CH₂ –CH₂– CH₂ – COOH

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• α-amino acid have the maximum importance because they are

the constituent units of proteins.

• Common system of nomenclature is still more popular in

naming amino acids and proteins.

• It is mostly α-amino acids, which are involved in the

building up of protein molecules.

• The simplest amino acid is α-amino acetic acid also called as

glycine. They are represented by the general formula :

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Where R is an alkyl or aryl group.

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Classification (acidic, basic and neutral)

Amino acids are classified into three groups.

• Neutral amino acids:

The amino acids which contain one amino and one carboxylic acid group are called as neutral amino acids.

Ex. Glycine (Amino acetic acid) H₂N– CH₂– COOH

b) Acidic amino acids:

The amino acids which contains one amino group and two

carboxylic groups are called as acidic amino acids. Ex. α- amino

succinic acid

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c) Basic amino acids:

The amino acids which contain two amino groups and one carboxylic groups are called as basic amino acids. Ex. α, δ - diamine n- valeric acid

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Dipolar nature of amino acids :

The – COOH group being acidic and ionizes in water solution

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The amino (– NH₂ ) group has lone pair electrons. It is a Lewis

base and capable of accepting a proton.

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In the neutral amino acid solution, – COOH group loses a proton

and the – NH₂ group of the same molecule accepts it.

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• The resulting ion is dipolar, charged but over all electrically

neutral. This is called as Zwitter ion.

• Therefore amino acids are amphotic.
• Their structure allows them to donate or accept proton while they

are dissolved, depending on the pH of the medium.

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• The pH at which the neutral form (II) predominate is dependent upon

the ionization constants of the amino group and the carbonyl group.

• It is the characteristics of a given α - amino acid and depends on the

nature of R.

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Iso electric point

When an ionized amino acid is placed in an electric field, it will actually migrate towards the opposite electrode.

Depending upon the pH of the medium:

• The positive form (II) will migrate towards cathode.
• The negative form (III) will migrate towards anode.
• The neutral form Zwitter ion will not migrate.

The pH at which the amino acids show no tendency to migrate when placed in an electric field is known as its isoelectric point.

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Methods of Preparation of α -amino acids :

• From α -halo acids

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Gabriel’s Phthalimide Synthesis

Strecker’s Synthesis

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Chemical Properties of α -amino acids :

Reactions due to –NH₂ group

• Formation of salt:

• Alkylation

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• Acylation

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• Reaction with nitrous acid

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• Condensation

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Reactions due to –COOH group

• Salt formation due with base

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• Ester formation

When amino acetic acid reacts with ethyl alcohol in presence of dry HCl, then ethyl amino acetate is formed.

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• Decarboxylation

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• Reduction

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Reactions due to both –NH₂ and –COOH groups

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Thank you