PRECIPITATION OF PROTEINS BY INORGANIC SALTS 16? 



the protein and polypeptide structure. Dehydration of a protein may 

 result in one or more of the following series of reactions: 



NHs.OOC.R.NHsOH 



+ H 2 O 



HOHsN 3OOH 



NHs.OOC.R.NHsOH /NHOC.R.NH 3 OH 



+ H 2 O 



NHOC.R.NHaOH /NHOC.R.NH 2 



= R \ + H2 



NHOC.R.NH 2 /NHOC.R.NH 



= R + H2 



and hydration, of course, may result in the reversion of this series of 

 changes. 



That proteins may be thrown out of solution in two very different 

 conditions of hydration is very clearly shown by the following experi- 

 ments : 



Anhydrous Casein dissolves readily in cold anhydrous Formic Acid; 

 still more readily in hot formic acid. If, to a two per cent, solution of 

 casein in formic acid, we add a fairly concentrated solution of Cupric 

 Chloride, the mixture is at first green, indicating the presence of lower 

 hydrates of cupric chloride, but on adding more of the solution it 

 becomes blue, and simultaneously with the appearance of a pure blue 

 color, but not before, precipitation of cupric caseinate occurs. If, 

 to five c.c. of a two per cent, solution of casein in formic acid, we add 

 1J, 2 or 2J c.c. of a saturated solution of cupric chloride, no precipita- 

 tion of the caseinate occurs, but on diluting this mixture with water a 

 precipitate results, and the appearance of this precipitate coincides 

 with the attainment of a clear blue color on the part of the mixture. 



About six cubic centimeters of water are required to produce a 

 permanent precipitate. This precipitate redissolves on heating, and 

 the mixture simultaneously becomes green; on cooling the blue color 

 reappears and with it the precipitate. If formic acid be added to the 

 mixture the precipitate redissolves as soon as the mixture becomes 

 green. If the precipitate be very slight it will redissolve on adding 

 alcohol. It cannot be urged that the formation of cupric caseinate 

 requires the presence of more cupric ions than are present in green 

 solutions, because green solutions of cupric chloride contain an abun- 

 dance of ions, and casein will react with very small amounts of metal 



