Introduction
The amino acids are the building blocks of Proteins
Proteins are the fundamental structural components of the body
Enzymes
Contractile proteins in the muscles to permit movement
Collagen in the bones
Plasma albumin, hemoglobin in blood
Immunoglobin to destroy infectious bacterial and viruses
Storage proteins (ferrin that bind to iron and help in its storage)
Maintain the osmotic pressure
Introduction
More than 300 different amino acids are known in nature
Only 20 are commonly found as constituents
of mammalian proteins
Amino acids structure
• At physiologic PH= 7.6 the amino acids
exists as Zwitterion
• The carboxyl group dissociate to form coo –
• The amino group is protonate to form NH3+
Zwitterion
Protein formation
N terminal C terminal
Physical properties
Colorless
Crystalline
More soluble in water than in polar solvents
They have high melting points more than 200
C
Essential amino acids: Non-essential amino acids
Cannot be
synthesized by the body
Must be taken from the diet
Ex: Valine, leucine, isoleucine,
phenylalanine, and tryptophan
Can be made by the body
Nutritional classification of amino
acids
Classification according to amino acids side
chain
Qualitative Tests
for Amino Acids
1- Ninhydrin Reaction:
A color reaction given by amino acids and peptides on heating with the chemical
ninhydrin.
The technique is widely used for the
detection and quantitation (measurement) of amino acids and peptides.
Ninhydrin is a powerful oxidizing agent
which reacts with all amino acids between pH
4-8 to produce a purple colored-compound.
1- Ninhydrin Reaction
The reaction is also given by primary amines and ammonia but without the liberation of Co
2
The amino acids proline and hydroxyproline also reacts but produce a yellow color.
Alpha-amino acid + 2 ninhydrin ---> CO
2+
aldehyde + final complex (purple) + 3H
2O
1- Ninhydrin Reaction
Method:
1 ml AA + 1 ml NH---- heat in boiling WB for 5min---Purple color
2-Xanthoproteic Reaction:
This reaction is specific for aromatic amino acids
This reaction involves the nitration of benzene nucleus in alkaline medium
L-Tryptophan
L-Phenylalanine L-Tyrosine
2-Xanthoproteic Reaction:
Aromatic AAs form yellow nitro derivative
on heating with conc. nitric acid, the salts of these derivatives are orange.
Nitrated tyrosine (a) and tryptophan (b)
2-Xanthoproteic Reaction:
Method:
1 ml AA + 1 ml conc. HNO3--- heat the
mixture in WB for 30s--cool--add drop-wise
40% NaOH to render the solution alkaline---
Yellow to orange color.
3- Millon Reaction:
This reaction is used to detect the presence of phenol (hydroxybenzene) which reacts with Millon's reagent to form red complexes.
- The only phenolic AA is tyrosine.
L-Tyrosine
3- Millon Reaction:
Method:
1 ml AA + 5 drops of Millon reagent --- heat the mixture in BWB for 10min--cool too room temp--add 5 drops of NaNO2---Brick red color.
4- Hopkin-cole Reaction:
This reaction is used to detect the presence of indol group
The indol group of tryptophan reacts with glyoxalic acid in the presence of conc. H2SO4 to give purple color.
L-Tryptophan
4- Hopkin-cole Reaction:
Method:
1 ml AA + 1 ml Hopkin-cole reagent ---mix
well--Carefully pour conc. H2SO4 down the side of the tube so as to form two layers --Purple
ring at the interface.
5- Sulfur Test:
This reaction is specific to detect the presence of sulfur
-The sulfur of cystein and cystine is converted to inorganic sulfide with conc. NaOH. Lead acetate is added and a ppt of black lead sulfide indicates a +ve reaction.
L-Cystein
5- Sulfur Test:
Method:
2 ml AA + 1 ml 40% NaOH + 1-3 drops of lead acetate solution--- heat the mixture in WB for 3min ---cool--observe any
change --- Black ppt.
6-Sakaguchi Reaction:
This reaction is used to detect the presence of guanidine group.
-The only AA that contains guanidine group is arginine which reacts with α-naphthol and an
oxidizing agent such as bromide water to give a red color
L-Arginine
6-Sakaguchi Reaction:
Method:
2 ml AA + 1 ml 2M NaOH + 1 ml ethanolic
0.02% α-naphthol --- mix wellcool in ice---
add 1 ml of alkaline hypochlorite solution----
Red color
