Xanthoproteic reaction
Encyclopedia
The xanthoproteic test is a method that can be used to determine the amount of protein soluble in a solution, using concentrated nitric acid
Nitric acid
Nitric acid , also known as aqua fortis and spirit of nitre, is a highly corrosive and toxic strong acid.Colorless when pure, older samples tend to acquire a yellow cast due to the accumulation of oxides of nitrogen. If the solution contains more than 86% nitric acid, it is referred to as fuming...

. The test gives a positive result in those proteins with aminoacids carrying aromatic groups, especially in the presence of tyrosine
Tyrosine
Tyrosine or 4-hydroxyphenylalanine, is one of the 22 amino acids that are used by cells to synthesize proteins. Its codons are UAC and UAU. It is a non-essential amino acid with a polar side group...

. If the test is positive the proof is neutralized with an alkali
Alkali
In chemistry, an alkali is a basic, ionic salt of an alkali metal or alkaline earth metal element. Some authors also define an alkali as a base that dissolves in water. A solution of a soluble base has a pH greater than 7. The adjective alkaline is commonly used in English as a synonym for base,...

, turning dark yellow.

This chemical reaction is a qualitative test, determining the presence or absence of proteins. To quantify, it is used another reaction, such as the Biuret
Biuret test
The biuret test is a chemical test used for detecting the presence of peptide bonds. In the presence of peptides, a copper ion forms a violet-colored complex in an alkaline solution...

, and an analysis is made by photometric spectrum.

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