Transcript tests on amino acids and proteins

TESTS ON AMINO ACIDS
AND PROTEINS
1. Ninhydrin Reaction
• Spot the solutions of amino acids provided
on a filter paper, add a drop of ninhydrin
reagent to each spot and hold the paper over
a bunsen flame to dry. Note and comment on
the colors produced.
• Expected results: Amino acids give purple
colors with ninhydrin reagent. But proline (an
imino acid) gives yellow color with ninhydrin.
2. Xanthoproteic Reaction
• Add 1 ml of conc. nitric acid to 1 ml of the
amino acid solution. Warm the mixture in a
boiling water bath and allow to cool. Note the
formation of precipitates and colors. Add
sufficient 40% NaOH to make the solution
strongly alkaline.
• Expected result: A yellow color in acid
solution which turns bright orange in alkali
indicates the presence of one or more of the
aromatic amino acids: tryptophan,
phenylalanine, and tyrosine.
3. Millon’s Reaction
• Add 5 drops of Millon’s reagent to 1 ml of the
test solution and warm the mixture in a
boiling water bath for 10 minutes. Cool to
room temperature and add 5 drops of sodium
nitrate solution.
• Expected result: The formation of a brick-red
color indicates the presence of a phenolic
amino acid, i.e., tyrosine or its derivatives.
Phenols and naphthols also give positive
results.
4. Violet Ring Test
• Add 2 ml of glacial acetic acid that has been
exposed to light to 2 ml of the test solution.
Pour 2 ml of conc. sulfuric acid carefully
down the sides of the test tube in a sloping
position so as to form two layers.
• Expected result: A violet ring at the interface
indicates the presence of tryptophan.
5. Nitroprusside Test
• Mix 0.5 ml of a fresh solution of sodium
nitroprusside (C5FeN6Na2O or Na2Fe(CN)5NO) with 2
ml of the test solution. Add 0.5 ml of ammonium
hydroxide. Note the color formed.
• Expected result: Thiol groups (cysteine) react with
sodium nitroprusside in the presence of excess
ammonia to give a red color.
6. Sagakuchi Reaction
• Mix 1 ml of 40% sodium hydroxide with 3 ml
of arginine solution and add 2 drops of αnaphthol. Mix thoroughly and add about 4
drops of bromine water. Note the color
formed.
• Expected result: The guanidine group of
arginine reacts with α-naphthol and an
oxidizing agent such as bromine water to
give a red color.
7. Biuret Test
• Biuret Test for Proteins: Biuret reagent is a light blue
solution which turns purple when mixed with a
solution containing protein. When the copper ions of
the Biuret Reagent react with peptide bonds in the
polypeptide chains, a purple color complex is
formed.
• To 2 ml of the test solution add 5 drops of biuret
reagent. Mix thoroughly and note the colors formed.
• This is a test for the presence of two or more peptide
bonds in solution. The intensity of the color
produced is proportional to the number of peptide
bonds present.
Determination of Protein
Concentration: Biuret Method
• You are provided
with protein
samples, e.g.,
serum. Place the
following reagents
into various test
tubes.
Test (ml)
Blank (ml)
Test Sample
0.1
-
3%Sodium
deoxycholate
0.4
0.4
Distilled
water
0.5
0.6
Biuret
reagent
4.0
4.0
• Mix the contents of the test tubes by gentle shaking
and let stand for 20 minutes for color development.
Then read the absorbance of each test solution
against the blank at 540 nm.
• You are also provided with a standard solution of
albumin (bovine serum albumin) with a
concentration of 10 mg/ml. Prepare a standard curve
using various concentrations of the albumin. This
can be done by replacing the test sample in the table
above with various amounts, i.e., 0.1, 0.2, 0.3, 0.4,
0.5, and 0.6 ml, of the albumin. Adjust the amount of
distilled water accordingly in order to keep the total
volume constant at 5ml. Read the absorbance of the
solution in each tube after 20 minutes and plot a
graph of absorbance against concentration of
albumin. Estimate the concentrations of the protein
samples provided from your graph.