CELL-CONTENTS AND CELL-WALLS. 39 



Dried powdered albumin may be bought ready for solu- 

 tion in water from chemical supply firms. 



(a) Biuret Reaction of Proteins. Add excess of 

 caustic soda (or potash) , then drop by drop some 1 per cent, 

 solution of copper sulphate a violet colour, which deepens 

 on heating. Compare with the rose pink colour given with 

 this test in the case of peptones ( 43). In making the 

 Biuret test, take care to use very little of the copper solu- 

 tion, adding it drop by drop, otherwise its blue colour 

 masks the reaction. 



(&) Iodine Reaction of Proteins. Add a little iodine 

 solution (see Appendix), and note the yellowish brown 

 colour given. Pour an equal volume of the iodine solution 

 into an equal volume of water in another tube, as a control, 

 and compare the colour with that given in the case of the 

 albumin. 



(c) Xanthoproteic Reaction. Add some strong nitric 

 acid a white precipitate which on boiling turns yellow. 

 Cool, and add strong ammonia the yellow precipitate 

 becomes orange. Instead of a coloured precipitate there 

 may be merely a yellow colour, but in any case this is a 

 good test for proteins. 



(d) Millon's Reaction. Add some Millon's reagent 

 (see Appendix)- a white precipitate, which on boiling turns 

 red. If very little protein is present in a tested liquid, no 

 precipitate but only a red colour may be given. Millon's 

 reaction is due to the presence of tyrosin in the protein 

 molecule. 



(e) Sulphur (Cystin) Reaction. Add a drop of lead acetate 

 solution, then caustic soda or potash sufficient to redissolve the pre- 

 cipitate first formed, and boil. A brown or black colour appears, 

 due to the separation of sulphuretted hydrogen from the amino acid 

 cystin in the protein molecule (unlike most amino acids, cystin 

 contains sulphur), this giving lead sulphide with the lead acetate. 



(/) Tryptophane ( Adamkiewicz) Reaction. Add excess of 

 glacial acetic acid to the solution, then (using a thistle tube) run in 

 strong sulphuric acid to the bottom of the test-tube. Gently shake 

 the tube, or simply let it stand for several minutes ; at the junction 

 of the liquids there appears a violet colour, which gradually spreads 

 through the solution. This reaction is due to the amino acid tryp- 

 tophane present in the protein molecule. 



