Test for Protein deals with the details of protein including its structure, properties, classification, and function as well as the details of different tests to detect proteins in a sample. The identification test for protein explains the principle and theory of each test, along with the materials and apparatus required, the procedures, observation, inferences, and discussion of the results of the tests.
Protein is an important biomolecule that is made up of one or more amino acid chains. Plants and animals synthesize these biomolecules and obtain them by consuming the plants. In this article, we will look into the meaning, theory, procedure, observation, results, and precautions of tests for proteins.
Table of Content
What are Proteins?
Proteins are highly complex large molecules that consist of many amino acids. These amino acids are connected to form long chains like beads arranged on a string. Twenty different amino acids naturally occur in proteins which are present in all living beings. Proteins have high nutritional value and are involved in various chemical and biological processes essential for life. Therefore, proteins are also known as the ‘building block of the body’.
Test for Protein to Confirm its Presence
The tests used to confirm the presence of protein in a sample are listed below.
- Biuret Test
- Xanthoproteic Test
- Millon’s Test
- Ninhydrin Test
Aim of the Protein Test
The test for protein aims to conduct different experiments to determine the presence of protein in any given sample like animal, plant, or food samples.
Theory of the Tests for Protein
Proteins are large biomolecules having long-chain polymers composed of amino acids. The main constituents of proteins are carbon, hydrogen, nitrogen, oxygen, and in some cases phosphorus and sulfur.
The following tests are conducted to detect the presence of proteins in a given sample.
- Biuret Test: In this test when a sample containing protein is treated with an alkaline solution and a drop of aqueous copper sulfate is added, a bluish-violet color appears. It is a qualitative test of protein.
- Xanthoproteic Test: If a sample containing proteins is treated with nitric acid, a yellow precipitate is formed. But when this solution is treated with alkali, it turns orange.
- Millon’s Test: The phenolic group present in proteins reacts with mercuric sulfate in the presence of sodium nitrite and sulfuric acid and turns the color of the solution red. Initially, when the sample containing protein is treated with Millon’s reagent, a white precipitate forms. Then after boiling, it turns into a brick-red colour.
- Ninhydrin Test: Proteins react with ninhydrin solution and turn it from a deep blue color to violet-pink or sometimes red color. Ninhydrin solution is prepared by dissolving 0.1gm of ninhydrin in about 100ml of distilled water.
Apparatus Set up and Chemicals Required for Protein Test
The following apparatus setup and materials are required to conduct the tests.
- Stirrer
- Dropper
- Water bath
- Distilled water
- Bunsen burner
- Test tube holder
- Clean and dry test tubes
- Samples to be used for testing
The following chemicals are used to conduct the tests
- Nitric acid
- Sulfuric acid
- Sodium nitrite
- Mercuric sulfate
- Pyridine solution
- Ninhydrin reagent
- Sodium hydroxide
- Copper sulfate solution
- Biuret reagent solution or solution of copper sulfate and caustic soda.
Procedures of Test for Protein
The following test procedures are used:
Biuret Test for Protein
The steps of Biuret test are as follows.
- A clean and dry test tube should be taken.
- The sample is taken with the test tubes.
- 2ml of sodium hydroxide and 5 to 6 drops of copper sulfate solution are to be added.
- Then the test tube is shaken gently to mix the ingredients thoroughly and allow the mixture to stand for 4 – 5 minutes.
- If a bluish-violet color appears, it indicates the presence of protein.
Xanthoproteic Test
The steps of this test are given below.
- The sample is taken in a clean and dry test tube.
- A few drops of concentrated sulfuric acid are added and shaken the test tube.
- The test tube is heated gently on a Bunsen burner.
- If yellow precipitate forms, then it confirms the presence of protein
Millon’s Test
The steps of Millon’s test are as follows.
- 2-3 drops of Millon’s reagent are added to a test tube containing the sample and shaken well.
- At first a white precipitate forms which on heating turns into a brick red colour. This indicates the presence of protein.
Ninhydrin Test
The steps of the ninhydrin test are as follows.
- 1-2ml of ninhydrin solution is added to a test tube containing the sample and shaken it.
- Then the mixture is boiled.
- A deep blue or purple color appears, which confirms the presence of protein.
Observation and Inference
The inferences that can be drawn from the observation of the experiments are listed below:
- Biuret test: A bluish-violet color of the solution indicates the presence of proteins.
- Xanthoproteic test: The formation of yellow precipitate confirms the presence of proteins.
- Millon’s test: A brick-red color of the resultant solution confirms the presence of proteins.
- Ninhydrin test: A deep blue or purple color of the resultant mixture indicates the presence of proteins.
Results and Discussion of Test for Protein
Based on the findings of the tests it can be determined whether the sample contains any protein or not.
Precautions
Some precautions should be taken while doing the tests to minimize errors. Some precautions are listed below:
- Droppers should be used to take the reagents from the bottles to prevent wastage of reagents.
- Lab aprons and hand gloves should be used during the experiment to minimize contamination.
- Chemicals should be handled with care throughout the experiment to minimize the waste of chemicals.
- The lab manual should be read thoroughly before starting the experiment.
- Caution should be taken while handling chemicals and acidic solutions to prevent accidents.
Conclusion – Protein and Test for Protein
The proteins are most abundant organic compounds that are found in almost all living organisms. They are essential entities for most of the biological functions. The test for protein in various sample requires different reagents and tests such as sodium hydroxide and copper sulfate (for Biuret test), concentrated sulphuric acid (for Xanthoproteic test), Million’s reagent, and Ninhydrin solution. The appearance of distinct colors in the test confirms the presence of proteins in the sample. The identification test for protein gives an idea about the amount of protein present in the sample.
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FAQs on Protein and Test for Protein
What Reagent is used to Test for Proteins?
Ans. The Biuret reagent is used to test for proteins. The Biuret reagent is a blue solution that contains allophanamide (biuret), cupric sulfate, and sodium hydroxide.
What is the Positive Test for Protein?
A positive result for the Biuret test is a purple color. A deep blue or purple color in the Ninhydrin test is a positive result.
What is Protein Analysis Used for?
Protein analysis is useful in understanding human physiology as well as for diagnosis, treatment, and disease monitoring. Protein analysis techniques can determine the quantity, purity, expression, structure, identification, and location of proteins.
What is the Overview of a Protein?
Proteins are large biomolecules consisting of one or more long chains of amino acid residues. They mainly consist of carbon, hydrogen, nitrogen, and oxygen. They are essential for various biological functions in almost all organisms.
What are the Tests for Protein?
The tests for protein include the Biuret, Xanthoproteic, Millon’s, and Ninhydrin tests, which produce specific color changes indicating protein presence.
What is the Biuret Test for Proteins?
The Biuret test for protein is used to detect proteins by forming a bluish-violet color in the presence of peptide bonds. It involves adding a solution of copper sulfate and sodium hydroxide to the sample, resulting in a color change if proteins are present.