Introduction of amino acid

Introduction of amino acids

Simply amino acids are the buildings blocks of proteins and are the organic nitrogenous compound that contains amino(-NH2)group and carboxyl(-COOH)group and therefore contain both acidic and basic properties.

Proteins are the most abundant and functionally diverse molecules in the living systems and every life process depends on this class of molecule ,that s why the building blocks of protein ie.amino acid is a very important class of molecule to conduct the life of the living system.

As the amino acid contain both acidic carboxyl group and basic amino group it can show both properties and it perform the reaction of carboxyl group and the amino group.The general representation of amino acid is given below:

The alpha carbon in all amino acid is asymmetrical except Glycin. Due to the asymmetric nature of amino acid, they exist in two optically active form designated as D and L. where D stands for dextrorotatory and L stands for Laevorotatory.These two are the mirror image of each other. The two forms in each paired are termed as stereoisomers/optical isomer/enantiomers.

D-form

Those amino acid having the amino group on the right side.

L-Form:

Those amino acid having amino group the left

Where n=number of asymmetrical or chiral carbon present in the moleculAlthough more than 300 amino acids have been described in nature ,only 20 amino acids are commonly found as constituents of mammalian proteins ie ,they are only amino acids that are coded by genetic code and these amino acids are termed as magic 20 amino acids. But now days 2 amino acid namely Selenocysteine and pyrrolysine have also been discovered as standard amino acids. Research are going on for further investigation of new standard amino acids.

Properties of amino acids

The different properties of amino acids are described as

• physical properties
• Chemical properties

A)Physical properties;

The physical properties of amino acids are described below:

• Colour;colorless
• State:They are crystalline substances and crystal may vary from niddle (Tyrosine)to thick hexagonal state (cysteine)
• Taste: Amino acid may be either tasteless( Tyrosine,Leucine),sweet(Glycine,Alanine and Valine)and bitter(Arginine)
• Melting Point /solubility :Not all amino acids are soluble in water but most are soluble depending on their molecular structure .Solubility is greatly affected by pH, temperature. They are least soluble at their isoelectric point. Polar amino acids are soluble in water wheres nonpolar amino acids are insoluble in water.
• Isoelectric point: The isoelectric c point (PI) is the pH at which an amino acid is electrically neutral ie. at this point the sum of positive charge is equal to the sum of negative charge .At this point, the net charge is zero and hence there is no migration of acid in an electric field.

For amino acids containing only two dissociable hydrogens at this condition, the PI is the average between the Pk values of these groups ie.

Isoelectric point (PI)=Pk₁+Pk₂

For example Alanine

Pk₁=2.3( acidic)

Pk₂=9.1 (basic)

ThereforePI=2.3+9.1/2=11.4/2

=5.7

• Amphoteric Properties( electrochemical property)

Amino acids contain both acidic and basic group so they have amphoteric property. In ionic form the proton migrates from carboxyl group to amino group with the production of carboxylate ion (-COO)- and ammonium (NH3)+

NH⁺₃+-CHR-COO^-+HA↔ NH⁺₃-CH-COOH+A^-

(aminoacid) (acid) (Ammonium salt) anion

NH⁺₃+CHR-COO^-+ BOH↔NH2-CH-COO+H₂O+B

Thus in acid solution COO ion acquires a proton and the amino acids become an ammonium salt of an acid, conversely, in alkaline solution, the ammonium ion donates a proton and the amino acids become the anion of salts. The both reactions are reversible in nature and depends on pH of medium and temperature. This property of amino acid is known as amphoteric property.

• Zwitterionic effect /Zwitter ion/Dipolar ion

The name Zwitter is derived from the German word which means hybrid. Zwitter ion is the hybrid molecules containing positive and negative ionic groups .Each amino acid is a nitrogenous compound having both an acidic carboxylic(-COOH)and a basic amino (-NH2 ) group. Two remaining positions of alpha-carbon are occupied by hydrogen and a hydrocarbon chain.

Zwitterionic property of amino acid is due to its dipolar nature .The amino acid at low pH condition amino group accept the proton and exist as positive charge ie. cation where as at high pH condition ,carboxyl group loose proton and exist as negative charge ie. anion .But at the isoelectric point, it bears both positive and negative charge .The carboxyl group of amino acid molecule donates the proton to an amino group of the same molecule to form a dipolar ion of an amino acid called as Zwitter ion. In Zwitter ion net charge is zero. the amino acid is capable of reacting with both acid and base due to its zwitter ionic property.H

Fig:Existence of amino acid as zwitterion

B) Chemical properties of amino acid ;

Amino acids contain both- NH2 and-COOH group and hence they exhibit the properties of acidic and basic reactions based on the involvement of the group in chemical reactions. There action are categories into 3 groups:

• Reaction involves in COOH group
• Reaction involves in NH2 group
• Reaction involves in NH2 and COOH group

• Reaction involves in NH₂groups
• Reaction with mineral acid

When amino acid reacts with mineral acid example HCL,amino acid hydrochloride is formed

R-CH(NH₂₎-COOH+HCl→R-CH(NH2.HCL)-COOH

Amino acid Amino acid hydrochloride

• Reaction with formaldehyde^;

When the amino acid reacts with formaldehyde, N-mono methylol derivative is formed which when reacts with a 2nd molecule of formaldehyde then N-dimethylol derivative is formed.

R-CH(NH₂)-COOH+HCHO→R-CH(NH-CH₂OH)-COOH→R-CH[N(CH₂OH)-CH₂OH]-COOH

Aminoacid formaldehyde N-monomethyl Ol N-dimethyl OL

Derivative derivative

• _{Reaction with nitrous acid:}

^When the amino acid reacts with nitrous acid then there is a production of alpha hydroxy acid with there a release of a water molecule and production of nitrogen gas.

R-CH(NHH )-COOH+HO-N=O→R-CH(OH)-COOH+N2+H

Aminoacid nitrous acid alpha hydroxy acid

• Reaction with1-fluro2-4dinitrobenzene(FDNB)

When the amino acid reacts with FDNB then dinitrobenzene ammino acid is formed which has the yellow colour.

fig: Reaction with 1-fluro, 2,4 dinitrobenzene

B)Reaction involves in COOH group:

• Reactions with alkalis^:

When carboxylic group of amino acid reacts with alkali (NaOH)then salt of amino acid is formed with the release of water molecules

-H2O

R-CH(NHH )-COOH+HONa→ R-CH( NH2)-COONa

Amino acid salt of amino acid

• Reactionswith alcohol:

When amino acids react with alcohol,then the product is ethyl esters of an amino acid is formed with the release of the water molecule.

R-CH(NHH)-COOH+HOC2H5→R-CH( NH2)-COO-C2H5

Aminoacids ethyl ester of amino acid

• Reaction with amine^:

When an amino acid reacts with amine then amide amino acids is formed with the removal of the water molecule.

R-CH(NH2)-COOH+HNHR’→R-CH(NH2)-COOHR’

Aminoacid amine amide aminoacid

C)Reaction involves in both COOH and NH2 group:

• Ninhydrin test^;

This is a general test and thus given by all amino acids. This test is due to a reaction between a amino group of free amino acids and ninhydrin .Ninhydrin is a powerful oxidising agent and in its presence aminoacids undergoes oxidative deammination liberating ammonia , CO₂ a corresponding aldehyde and reduced form of Ninhydrin( Hydrindantin) to give a blue substance diketohydrin (Ruhemanns complex). However, in case of amino acid like proline and hydroxyproline, a different product having a bright yellow colour is formed. Asparagine, which has a free amide group reacts to give a brown colour product.