Xanthoproteic Test

Demonstration Experiment on Video

Xanthoproteic Test

Objective: Test for Aromatic Rings in Proteins

Peter Keusch

German version

Chemicals:
alanine
tyrosine
egg-white
HNO₃ 65%

Glass wares:
3 conical measures, graduated, 250 mL
3 beakers 250 mL
1 beaker 50 mL
2 snap-cap vials 10 mL
3 glass stirring rods

Hazards and safety precautions:

Nitric acid may be fatal if swallowed or inhaled. Extremely corrosive. Contact with skin or eyes may cause severe burns and permanent damage.

Safety glasses and protective gloves required. The experiment should be performed under a portable fume cupboard giving all-round visibility!

Experimental procedure:

Three conical measures are set up as described in the table below. The solutions are mixed with a glass stirring rod.

conical measure 1	100 mL of boiling water	0.05 g of alanine	100 mL of HNO₃ 65 %
conical measure 2	100 mL of boiling water	0.05 g of tyrosine	100 mL of HNO₃ 65 %
conical measure 3	100 mL of boiling water	20 mL egg-white	100 mL of HNO₃ 65 %

Results:

In conical measure 2 a yellow color can be seen clearly. An intensely yellow colored precipitate does appear in measure 3. The solution in measure 1 remains colorless.

Video clip (Download RealPlayer .rm file)

Discussion:

Using 65% nitric acid the aromatic rings of amino acids like tyrosine and tryptophan are nitrated. The nitro derivate show an intensely yellow color. Because nearly all proteins contain aromatics it is taken as a protein-test either.

Nitrated tyrosine (a) and tryptophane (b)

The yellow stains on the skin caused by nitric acid are the result of the xanthoproteic reaction. The epidermis cells of the skin contain aromatic proteins.

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