GENERAL CHEMICAL CHARACTERS OF PROTEINS 57 



the precipitate obtained by adding copper salts to a solution of egg- 

 white. Harnack, by precipitating in the presence of excess of pro- 

 tein, obtained a precipitate containing 1*35 per cent. Cu ; when, 

 however, he precipitated in presence of excess of the copper salt, he 

 obtained a product containing 2^64 per cent. Cu. Other investiga- 

 tors have obtained products in which the percentage of copper varied 

 between 07 and 4*15 per cent. Equally unsatisfactory and variable 

 are the results obtained by the precipitation with salts of other heavy 

 metals. 1 



The formation of the precipitates in question has more recently 

 formed the subject of investigations by Galeotti and by Pauli, who 

 have called attention to the complexity of the question. 



Galeotti noticed that if a 3 per cent, albumin solution be allowed 



to fall, drop by drop, into an copper sulphate solution, a thick pre- 

 cipitate first forms, which redissolves on the addition of an excess 

 of protein, so that finally a homogeneous solution is obtained. If 

 more copper sulphate be added to this clear solution a precipitate is 

 formed again, which will redissolve on addition of excess of protein. 

 If an albumin solution be added, drop by drop, to a concentrated 

 copper solution, a precipitate will form as the drops first enter the 

 salt solution, but will disappear on shaking ; on addition of more 

 albumin a precipitate continues to be formed, which dissolves less 

 and less readily after each addition, until finally it becomes per- 

 manent. If, on the other hand, concentrated copper sulphate be 

 added to a protein solution a precipitate is formed, which gradually 

 dissolves on the addition of excess of the salt, giving a clear green 

 solution which contains protein in large quantities. 



Silver salts behave in a somewhat different way. If an albumin 

 solution be allowed to drop into a I per cent, silver nitrate solution 

 a flocculent precipitate forms, which redissolves in an excess of pro- 

 tein. If, on the other hand, a solution of silver nitrate be allowed to 

 fall into an albumin solution, no precipitate forms until a certain 

 definite quantity of the salt has been added ; at this point a pre- 

 cipitate commences to form, which increases in quantity with each 

 additional drop of the salt. If albumin be added again, the pre- 

 cipitate redissolves. 



Galeotti draws from his investigations the following conclusions : 

 I. Proteins form no definite compounds of constant composition 

 with salts of the heavy metals ; the precipitates which form when 

 protein solutions and solutions of salts of the heavy metals are 

 mixed are simply loose compounds, the composition of which 

 depends on the conditions of precipitation. II. The precipitation 

 phenomena are often reversible, 2 in the sense that the precipitates 

 are soluble in excess of either reagent. III. The composition of 

 the precipitate depends on the composition of the supernatant liquid 

 and is determined by the thermodynamical laws of chemical equili- 

 brium. IV. For the system albumin (egg or serum), water and salt 



1 A table of these results is given in Galeotti's paper. References to the earlier 

 literature are given in Harnack's paper. 



2 Galeotti uses the expression reversible, but only in the sense described in the 

 text, i.e., solubility on adding excess of either reagent. The reversibility is to be dis- 

 tinguished from that of the precipitation by, e.g., salts of alkalis, where the precipitates 

 redissolve on addition of water. 



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