Perform the precipitation reactions on the given protein solution and record your observations.
How to lay out the answer. Follow the frame, then write it in your own words.
1 · Aim. To study the precipitation of the given protein by heat, at the isoelectric pH, by salting out, by heavy metal salts, by alkaloidal reagents and by alcohol.
2 · Principle.
Charge and a water shell keep proteins dissolved.
Heat denatures; the isoelectric pH removes net charge (casein pI about 4.6).
Ammonium sulphate removes water: globulins at half, albumins at full saturation.
Heavy metal cations bind protein anions; alkaloidal anions bind protein cations; alcohol dehydrates.
3 · Requirements. Egg albumin in saline, casein or milk; 1% acetic acid, ammonium sulphate (saturated and solid), lead acetate, mercuric chloride, copper sulphate, picric acid, tannic acid, TCA, 95% ethanol; test tubes, water bath, filter paper.
4 · Procedure.
Heat the upper part of 5 mL albumin; add 1-2 drops acetic acid if needed.
Add 1% acetic acid to casein to about pH 4.6.
Half-saturate with ammonium sulphate, filter, then fully saturate the filtrate.
Add metal salts to neutral protein and acid reagents to slightly acidified protein; add ethanol to protein.
5 · Observation. [Table: Test | Observation | Inference for all seven tests]
6 · Result. The protein was precipitated by heat, at its pI, by salting out (globulin at half, albumin at full saturation), by heavy metals, by alkaloidal reagents and by alcohol.
7 · Precautions. Gloves and goggles; heavy metal salts are poisons; little acetic acid; full saturation with solid salt; never pipette by mouth.
Scoring tip: For each test, say why the protein comes out (charge, water shell or denaturation). That one line of reason is what separates a 6 from a 10.
