12 CHEMISTRY OF THE PROTEIDS CHAP. 



The last of these, recommended by Ehrlich, is more delicate than the 

 other two, for it will allow tryptophane to be recognised in dilutions 

 of 0.003 per cent. 



According to Rhode tryptophane is the only radical present in 

 albumins which gives colour-reactions with the aromatic aldehydes. 



To obtain colour-reactions with albuminous substances proceed 

 thus : Add to the albuminous solution 10 drops of a 5 per cent solution 

 of p-dimethyl-amino-benzaldehyde in 1 per cent sulphuric acid ; then 

 add strong sulphuric acid, shaking constantly, till the colour is 

 obtained. Vanillin is used similarly in a 5 per cent alcoholic solution, 

 while p-nitro-benzaldehyde is added in the solid form as it is insoluble 

 in acids, alkalies, and alcohols. 



When given to rabbits in the food, p-dimethyl-amino-benzaldehyde 

 appears in the urine x as a paired glycuronic acid, having probably the 

 formula 



(CH 3 ) 2 N C 6 H 4 CO CH (CHOH) 9 CH CHOH COOH, 



I o 1 



and also as p-monomethyl-amino-benzoic acid and as free p-dimethyl- 

 amino-benzoic acid. 



II. PRECIPITATION TESTS 



Generally speaking, albumins are only soluble in water, and there- 

 fore are precipitated by most other fluids. Amongst the latter the 

 most important is alcohol. In absolute alcohol all albumins are in- 

 soluble, but the percentage strength of alcohol required for precipitating 

 different albumins varies greatly with the individual albuminous sub- 

 stances and may serve to distinguish between albumins (Hofmeister, 2 

 Mann, 3 Tebb 4 ). That an albumin such as egg-white may show different 

 states of precipitation according to the strength of ethyl and methyl 

 alcohols and of acetone has been fully described by Mann. 3 



The chlorides and the sodium salts, especially of the denaturalised 

 albumins, are much more soluble in alcohol than are the albumins them- 

 selves. Urea and the alcohol soluble salts behave, according to Spiro, 5 

 as bases, for they increase the solubility in alcohol. Spiro found, 

 further, that the higher members of the alcohol series have an 

 increasing power of precipitating albumins. Of the aromatic alcohols, 



^ J M. Jaffe, Zeit. f. physiol. Chem. 43. 374 (1905). See also about dimethyl-amino- 

 antipyrin in Ber. d. deutsch. Chem. Ges. 34. 2737 (1901). 



2 F. Hofmeister. See p. 179 and table on p. 180. 



3 Mann, Physiological Histology, 1902, pp. 103-104. 



* 4 M. Christine Tebb, Journal of Physiology, 3O. 25 (1904). 

 5 K. Spiro, Hofmeister' s Beitr. IV. 300 (1903). 



