This nitration reaction, when used to identify the presence of an activated benzene ring, is commonly known as the xanthoproteic test, because the product is yellow.
Xanthoproteic comes from the Greek word xanthos, which means yellow. The intensity of the yellow colour deepens when the reaction occurs in basic solution.
This reaction is one of the reactions that occur if you spill a concentrated solution of nitric acid onto your skin.
The proteins in skin contain tyrosine and tryptophan, which become nitrated and turn yellow. Then the nitrated tyrosine complexes mercury I and mercury II ions in the solution to form a red precipitate or a red solution, both positive results.
Proteins that contain tyrosine will, therefore, yield a positive result. However, some proteins containing tyrosine initially forms a white precipitate that turns red when heated, while others form a red solution immediately. Both results are considered positive. Note that any compound with a phenol group will yield a positive test, so one should be certain that the sample that is to be tested does not contain any phenols other than those present in tyrosine.
The Hopkins-Cole test is specific for tryptophan, the only amino acid containing an indole group. The indole ring reacts with glyoxylic acid in the presence of a strong acid to form a violet cyclic product. The Hopkins-Cole reagent only reacts with proteins containing tryptophan.
The protein solution is hydrolyzed by the concentrated sulphuric acid at the solution interface. The nitroprusside test is specific for cysteine, the only amino acid containing a sulfhydryl group —SH.
The group reacts with nitroprusside in alkaline solution to yield a red complex. Top Menu BiologyDiscussion. Amino Acids: Nomenclature and Classification. This is a question and answer forum for students, teachers and general visitors for exchanging articles, answers and notes.
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Others Others. Other uncategorized cookies are those that are being analyzed and have not been classified into a category as yet. It is another color test for proteins that indicates the presence of amino acids in a solution. Only alpha amino acids give a positive Ninhydrin test. Ninhydrin is a powerful oxidizing agent that causes oxidative decarboxylation of the alpha amino acids forming an aldehyde, ammonia, and Hydrindantin reduced form of Ninhydrin.
This Hydrindantin reacts with ammonia and another Ninhydrin molecule to form a bluish-purple colored complex. The formation of bluish-purple color indicates the presence of free alpha amino acids in the solution.
The general tests listed above confirm the presence or absence of proteins in the given solution. Once the presence of proteins have been confirmed, the next step in the identification is to differentiate it from other proteins. For this purpose, differentiating tests are performed.
The solubility test is based on the ability of proteins to dissolve in water other than keratin. Keratin is a large fibrous protein having numerous disulfide bonds. These disulfide bonds make it insoluble in water while the rest of the proteins are highly soluble due to hydrophilic amino acids present in them. Some proteins coagulate upon heating like albumin and globulin. These are the two most common proteins present in plasma. Heat coagulation test is used to differentiate albumin and globulin from other proteins.
When proteins are exposed to heat or high temperature, they lose their structure and become denatured. These denatured proteins form a coagulum. The addition of acetic acid precipitates the coagulum by providing suitable conditions. A white coagulum is formed in the upper portion of the test tube.
The coagulum gets intensified upon adding the acetic acid solution. The white coagulum indicates the presence of a heat coagulable protein i. Salt saturation test is used to differentiate albumin from globulin.
Globulin undergoes precipitation upon half-saturation while albumin only precipitates upon full-saturation. Proteins are kept dissolved in solution by water molecules surrounding them. These water molecules make a hydration shell around the protein molecules.
When neutral salts are added to the solution, the ions present in the slat have more affinity for water molecules. They attract the water molecules, removing the hydration shell around the proteins, and rendering them insoluble. As a result, the proteins precipitate out of the solution. If a white precipitate is formed, it indicates that globulin protein is present in the solution. If filtrate form purple color on the Biuret test, it shows other proteins are also present that are not precipitated yet.
If the Biuret test is negative, it indicates that all the proteins present in the solution has been precipitated. If the filtrate gives a positive Biuret test, it contains additional proteins. The reverse is true if the Biuret test is negative for filtrate. These are the final tests that confirm the presence of a particular protein in the solution.
The following are confirmatory tests for some important proteins. This test has already been discussed. It is a differentiating test as well as a confirmatory test for albumin and globulin proteins. Isoelectric pH is defined as the pH at which amino acids carry no net charge and is electrically neutral. It has an equal number of positive and negative charges at this point.
It is a confirmatory test for mil protein, Casein. At isoelectric pH, protein has minimum solubility and tend to precipitate out. The protein molecules come together and form precipitate at this pH.
The addition of acetic acid to the solution of protein drops the pH to 4. It is a confirmatory test for keratin. The sulfur atoms present in cysteine and cystine residues are detected in this test. Keratin is rich in cysteine residues from string disulfide bonds between them rendering it insoluble in water.
When a solution containing keratin is boiled with sodium hydroxide, the sulfur present in its amino acids cysteine is converted to inorganic sodium sulfide.
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