Lab Activity 9 Precipitation Of Proteins

Report
Lab Activity 9
Precipitation Of Proteins
IUG, Spring 2014
Dr. Tarek Zaida
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Protein Precipitation
• Proteins are precipitated from solution by:
1. Certain acids some of which are alkaloidal
reagents such as picric-, and tannic acid
2. Salts of heavy metals such as HgCl2, AgNO3,
CuSO4, Pb(C2H3O2) etc.).
3. By conc. Solution of (NH4)2SO4 , Na2SO4, NaCl,
4. By dehydrating agents such as alcohol
(ethanol, methanol), acetone.
5. Isoelectric point.
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1. Effect of strong Acids and Alkali
•
•
•
•
Reagents:
1% solution of egg albumin,
conc. HNO3, H2SO4, HCl, and acetic acids,
conc. NaOH
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Procedure
1. Place 2 ml of conc. HNO3 in a test tube, inlcine
the tube and add the dilute albumin slowly from
a pipette, allowing the solution to run down the
side of the tube and form a layer over the nitric
acid.
Note the appearance of a protein precipitate at
the zone of contact between the two fluids.
2. Now mix the contents of the tube thoroughly by
careful shaking.
Is protein precipitated by conc. HNO3?
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3. Repeat the above steps using conc. H2SO4,
conc. HCl, acetic acid, and NaOH.
- Observe what happens in each case, then
compare it with the first experiment with
HNO3.
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2. Precipitation by Metallic Salts
• Reagents
• 1% Solution of egg albumin,
5% solution HgCl2, CH3COOPb,CuSO4,
• 1% solution of AgNO3
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Procedure
1. Prepare 4 test tubes, each containing 2 to 3
ml of dilute albumin sol., to the first test tube
add HgCl2 sol., drop wise slowly until an
excess of the reagent has been added.
2. Record your observations.
Unless the reagent is added very slowly, the
formation of the precipitate may not be
noted, due to it’s solubility in excess of the
reagent.
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• 3. Repeat the above exp. With CH3COOPb,
CuSO4, and AgNO3
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3. Precipitation by Alkaloidal Reagents
• Reagents
• 1% albumin solution, sat. sol. Of picric acid,
10% trichloroacetic acid solution (TCA), tannic
acid sol., Phosphotungstic acid sol.
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Procedure
1. Prepare 4 test tubes each containing 2 to 3 ml of
albumin sol.
2. To the first test tube add picric acid drop wise,
until an excess of the reagent has been added.
3. Note any change that may occur.
4. Repeat the exp. With Trichloroacetic acid sol.,
Tannic acid sol., phosphotungstic acid sol.
5. Are these precipitates soluble in excess of the
reagent? (acidify with hydrochloric acid before
testing with the last 2 reagents).
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4. Precipitation by K4[Fe(CN)6] · 3H2O
• Reagents
• 1% Albumin sol., 1% potassium ferrocyanide
sol.
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Procedure
1. To 5 ml of albumin sol., in a test tube add 5
to 10 drops of acetic acid, mix well and add
potassium ferrocyanide drop by drop, until a
precipitate forms.
2. Write down your observation
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5. Fractional Precipitation of Proteins by Conc. Salt
solutions
• Reagents
• Dilute egg white solution (mix 1 volume of raw
egg with 4 volumes of 1 % NaCl and filter).
• Sat. sol. of (NH4)2SO4, 1% acetic acid solution.
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Procedure
1. To a portion of the dilute egg-white sol. add an equal
volume of a saturated sol. of (NH4)2SO4 and mix. Does
the egg white contain a portion which precipitates by
half-saturated (NH4)2SO4?
2. If a precipitate is formed, then filter the content, and
to the filtrate add an excess of solid (NH4)2SO4 and stir
until the solution is saturated with salt.
3. Record what happens? If a precipitate formed, filter
and to the filtrate add 2 to 3 drops of 1% acetic acid
sol. And heat to boiling.
4. If there is no precipitate formed, it indicates the
absence of protein in the last filtrate.
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6. Precipitation of Proteins at the
Isoelectric Point
Reagents
• Casein powder,
• Solutions of: 1 N NaOH, 1 N CH3COOH, H2Od,
• Volumetric flask
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Procedure
1. Into a 50 ml volumetric flask add 20 ml of
water.
2. Add 0.25 g of pure casein, followed by the
addition of 5 ml of 1 N NaOH solution.
3. Once casein is dissolved, add 5 ml of 1 N
acetic acid solution, then dilute with H2O to 50
ml and mix well. The resulted solution is a 0.1
N casein acetate sodium.
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4. Setup a series of 9 test tubes as shown in the
table below.
5. In the first test tube put 3.2 ml 1 N CH3COOH,
and 6.8 ml H2O and mix thoroughly.
6. In each of the other test tubes (2-9) put 5 ml
H2Od.
7. From the test tube 1 transfer 5 ml to the test
tube 2, and mix thoroughly.
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8. Repeat step 7 for the rest of test tubes (3 - 9).
9. Now to each test tube (1 -9) add 1 ml of the
casein acetate sodium solution, and shake the
test tubes immediately.
10. Let the samples stand for 30 min, and note
the turbidity in the 9 test tubes.
11. Use + / – signs to describe the turbidity in
the different test tubes.
12. You should observe the most precipitation in
the test tube which has the pH around 4.7
(close to the isoelectric point of casein).
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TUBE
1
2
3
4
5
6
7
8
9
1N
CH3COOH
1.6
0.8
0.4
0.2
0.1
0.05
0.025
0.012
0.006
PH
3.5
3.8
4.1
4.4
4.7
5.0
5.3
5.6
5.9
TURBIDITY
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