The xanthoproteic reaction is a method that can be used to detect a presence of

protein Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residues. Proteins perform a vast array of functions within organisms, including catalysing metabolic reactions, DNA replication, res ...

soluble in a solution, using concentrated

nitric acid Nitric acid is the inorganic compound with the formula . It is a highly corrosive mineral acid. The compound is colorless, but older samples tend to be yellow cast due to decomposition into oxides of nitrogen. Most commercially available ni ...

. The test gives a positive result in amino acids carrying

aromatic In chemistry, aromaticity is a chemical property of cyclic ( ring-shaped), ''typically'' planar (flat) molecular structures with pi bonds in resonance (those containing delocalized electrons) that gives increased stability compared to satur ...

groups, especially in the presence of

tyrosine -Tyrosine or tyrosine (symbol Tyr or Y) or 4-hydroxyphenylalanine is one of the 20 standard amino acids that are used by cells to synthesize proteins. It is a non-essential amino acid with a polar side group. The word "tyrosine" is from the G ...

. If the test is positive the proof is neutralized with an

alkali In chemistry, an alkali (; from ar, القلوي, al-qaly, lit=ashes of the saltwort) is a basic, ionic salt of an alkali metal or an alkaline earth metal. An alkali can also be defined as a base that dissolves in water. A solution of a ...

, turning dark yellow. The yellow colour is due to

xanthoproteic acid Xanthoproteic acid is a noncrystallizable yellow substance derived from proteins upon treatment with nitric acid. Nitric acid reacts with proteins to form xanthoproteic acid. This reaction is known as the xanthoproteic reaction. This test is carr ...

which is formed due to

nitration In organic chemistry, nitration is a general class of chemical processes for the introduction of a nitro group into an organic compound. The term also is applied incorrectly to the different process of forming nitrate esters between alcohols an ...

of certain amino acids, most common examples being

and

tryptophan Tryptophan (symbol Trp or W) is an α-amino acid that is used in the biosynthesis of proteins. Tryptophan contains an α-amino group, an α-carboxylic acid group, and a side chain indole, making it a polar molecule with a non-polar aromatic ...

. This chemical reaction is a qualitative test, determining the presence or absence of proteins. Xanthoproteic reaction

Procedure

Add 1 ml of concentrated HNO₃ to 1 ml of the test sample. Heat the mixture and cool it. Slowly add

sodium hydroxide Sodium hydroxide, also known as lye and caustic soda, is an inorganic compound with the formula NaOH. It is a white solid ionic compound consisting of sodium cations and hydroxide anions . Sodium hydroxide is a highly caustic base and al ...

(40% w/v in water) solution until the mixture becomes alkaline and a colour change is noted. If the colour changes from

yellow Yellow is the color between green and orange on the spectrum of light. It is evoked by light with a dominant wavelength of roughly 575585 nm. It is a primary color in subtractive color systems, used in painting or color printing. In th ...

to orange, this indicates the presence of an

aromatic amino acid An aromatic amino acid is an amino acid that includes an aromatic ring. Among the 20 standard amino acids, the following are classically considered aromatic: phenylalanine, tryptophan and tyrosine. Although histidine contains an aromatic ring, ...

. When human skin or nails are exposed to nitric acid, they turn yellow after some time, indicating the presence of protein. The finger nails show a bright yellow colour (finger nails are made up of

keratin Keratin () is one of a family of structural fibrous proteins also known as ''scleroproteins''. Alpha-keratin (α-keratin) is a type of keratin found in vertebrates. It is the key structural material making up Scale (anatomy), scales, hair, Nail ...

, which is a protein) which cannot be scraped off, unlike the yellow colouration on the skin which can be peeled off.

References

{{Reflist Biochemistry detection reactions Protein methods

Procedure

See also

References