Protein gives protein precipitation tests because Proteins behave as cations in acid solutions and are precipitated with acid radicals to form insoluble salts. On the other hand, in alkaline solutions, they act as anions and are precipitated with heavy metals.By virtue of their sulphydryl (-SH) groups, they also act with salts of heavy metals.
APPARATUS:
Test tubes
A test tube holder
Test Tube Rack
Pipettes
Beaker
REAGENTS:
Protein solution
20% Salicyl-Sulphonic Acid
Esbach’s Reagent
10% Trichloroacetic Acid
4% Mercuric Chloride
2% lead acetate
2% Cupric Sulphate
5% Sodium Tungstate
Dilute H2SO4
Dilute NaOH
Precipitation of proteins from their aqueous solutions can be effected by several reagents, which are grouped as follows along with test procedure:
A. ACID OR ANIONIC PRECIPITANTS
1. SALICYL-SULPHONIC ACID:
Take 1 ml of original solution (protein solution) in a test tube.
Add 5 drops of 20% Salicyl-sulphonic acid.
** This reagent is of value for detection of traces of protein in the urine.
2. ESBACH’S REAGENT:
Take 2 ml of the original solution (protein solution) in a test tube.
Add 2 ml of Esbach’s reagent.
A yellowish precipitate is formed.
Esbach ’s Reagent is made by dissolving 10 gm Picric Acid, and 10 gm of Citric Acid in one litre of distilled water.
3.TRICHLOROACETIC ACID:
Take 1 ml of the original solution (protein solution) in a test tube
Add 1 ml of 10% trichloroacetic acid.
A white precipitate is formed.
Trichloroacetic acid is used to remove proteins present in the blood in certain quantitative estimations.
** This reaction is applied in the preparation of protein free filtrate (PFF) from blood or serum for most of the quantitative estimations.
4. SODIUM TUNGSTATE:
Take 2 ml of original solution (protein solution) in a test tube
Add 2 drops of dilute H2S04.
Add 0.5 ml of 5% sodium tungstate solution.
A white precipitate appears.
B. ALKALINE OR CATIONIC PRECIPITANTS
1. ZINC SULPHATE:
Take 3 ml of original solution (Protein solution) in a test tube.
Add 3 drops of dilute sodium hydroxide
And 1 ml of 4% zinc sulphate solution.
A white precipitate is appears.
C. PRECIPITATION BY HEAVY METALS
1. LEAD ACETATE:
Take 1 ml of original solution (protein solution) in a test tube.
Add 1 ml of lead acetate solution.
A white precipitate is appears.
2. COPPER SULPHATE:
Take 1 ml of original solution (protein solution) in a test tube.
Add 1 ml of 2% copper sulphate solution.
A white precipitate is formed.
3. MERCURIC CHLORIDE:
Take 1 ml of original solution (protein solution) in a test tube.
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