Xanthoproteic reactions

Xanthoproteic Reaction.—This reaction is produced by the action of concentrated nitric acid and results in a yellow color which turns orange on the addition of ammonium hydroxide. The yellow stain of nitric acid on flesh is due to the protein that is present. The reaction is caused by the presence of the phenyl group, (CcHb—), and any nonprotein compound containing this group also gives the test. 

Xanthoproteic Reaction - Addition of concentrated nitric acid to protein produces yellow colour on heating, the colour changes to orange when the solution is made alkaline. The yellow stains upon the skin caused by nitric acid are the result of this xanthoproteic reaction. This is due to the nitration of the phenyl rings of aromatic amino acids. 

Xanthoproteic Reaction. Add 1 ml of concentrated nitric acid to the solid protein and warm slightly. Apply the test to small amounts of solid gelatin, casein, and egg albumen. 

A yellow color with HNO, changing to orange with NH,HO (Xanthoproteic reaction). 

II. Xanthoproteic Reaction.—When proteins are treated with strong nitric acid a yellow coloration or yellow solid is formed. It is sometimes necessary to heat the solution to boiling to get a positive test. If an excess of sodium hydroxide is added to the solution, the yellow substance formed by the acid is changed in color to orange. The production of the color in this test is probably associated with the presence of a benzene ring in the compound. When many simple derivatives of benzene are treated with strong nitric acid, the nitro-substitu-tion-products formed have a yellow color. 

The keratins are the chief constituents of the epidermis and the horny substance found in mammals and birds they occur in hair, hoofs, horns, wool, and feathers, and are the most insoluble of all the albuminoids. The products of the hydrolysis of the keratin obtained from the horn of the ox are enumerated in section 685. In addition to those given, ammonia is obtained in relatively large amounts. The keratins contain a relatively high percentage of cystine human hair yields about 14 per cent of this acid. With nitric acid the keratins give a marked xanthoproteic reaction which is observed, for example, when the acid comes in contact with the skin. 

Amino acids having specific groupings which may be readily determined have offered a simple approach to protein analysis. Of these tyrosine (and tryptophan) have received the most attention. Little effort has been made to analyze for protein content by determination of the sulfur containing amino acids, cystine and cysteine. Amino acids containing a benzene ring react with hot concentrated nitric acid (xanthoproteic reaction) to form yellow nitro compounds which become orange on neutralization. Millon s reagent and that of Folin, particularly the... 

The determination of protein by nitration of its brazene ring containing amino acids (xanthoproteic reaction) has the single advantage of a ampler reagent than methods based on the Folin phenol reagent. The reaction is less sensitive and is somewhat difficult to standardize. Techniques for protein analysis by this means have been described by various authors (107,108,109). 

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