io 4 PRINCIPLES OF GENERAL PHYSIOLOGY 



arsenious .sulphide, do. It remains as yet uncertain whether many of the complex 

 proteins, occurring naturally, are not aggregates of various simpler ones, united 

 by means not strictly chemical. 



As remarked above, proteins may be either weak aoids or weak bases. In t In- 

 former case, owing to the preponderance in number of H - ions given off, the 

 colloidal ion is left negative, just as silicic acid is ; in the latter case, the protein 

 particle will be positive, as it gives off rapidly moving OH' ions. If the protein 

 is an aggregate, it is clear that dissociation will occur in the case of those molecules 

 on the surface only, and the colloidal ion will be more bulky than if the protein is 

 in single molecules. 



It is of interest to record the fact that crystals of leucine, a simple amino -arid, suspended 

 in their own saturated solution, have a small negative charge, as would be expected from the 

 slightly more acidic than basic character of leucine itself. When a solution of leucine is made 

 acid, there is a deposition of the solute on the cathode, when a current is passed through the 

 solution, and vice versa when made alkaline. Thus it behaves in the same way as a protein 

 under the same conditions. 



On addition of small amounts of a strong acid to a solution of a very weak 

 acid, by the law of mass action the dissociation of the weak acid is practically 

 abolished, so that its molecules are almost entirely present as neutral uncharged 

 elements. The same thing happens when a strong acid is added to a protein 

 solution. But owing to the amino acid nature of the latter, the effect in question 

 is replaced by formation of a salt when the quantity of acid added is increased : 

 the acid then combines with the basic groups of the amino-acid. At a particular 

 concentration of acid, therefore, the protein exists with a maximum of electrically- 

 neutral molecules. This is the isoelectric point, which varies with different 

 proteins, according to the degree of their acidic properties. 



Now it is found experimentally that the lyophile character varies greatly 

 according to the presence or absence of the electric charge, i.e., whether the 

 protein is in the form of an ion or otherwise (Pauli, 1912, p. '226). The increase 

 of hydration implied in this, goes with increase of properties such as viscosity, 

 imbibition, solubility, osmotic pressure, difficulty of coagulation by alcohol and 

 heat, surface tension, and rotation of polarised light. The importance of the 

 distribution of the solvent between the phases of a colloidal system has been 

 emphasised by Hatschek, as already mentioned, and we see now how the effect of 

 acid and of alkali in increasing the water content of the dispersed phase may be 

 explained by the production of protein ions. As will be shown in more detail 

 in Chapter VIII., ions are usually associated with a considerable number of 

 molecules of the solvent. 



The action of neutral salts is not so simple. The example of the Bence-Jones' 

 protein, given previously, points to a double action, if not a triple one. 



The effect of salts in large concentration in removing water from the internal 

 phase, and thus producing what is known as "salting out," has been described 

 under the head of emulsoids above (page 97). The precipitating power of salts 

 follows the " Hof meister series" and it is important to note that certain properties 

 of water, such as surface tension, viscosity, compressibility, are affected in the 

 same order, so that it is to be supposed that the action of salts in removing water 

 is exerted rather on the water itself than on the protein (Pauli, 1912, p. 238). 



The same series was found by Rothmund, independentlv (%<tt*ch. /". />/< //*//.. ('In in., 33, 

 401), to apply to the case of the solubility of phenylthiocarbamide. Schryver (1910) brings 

 the phenomena into relationship with the effect of the salts on the surface tension of water. 



Whether there is actual chemical union between proteins and neutral salts is a 

 matter of some dispute. It has been suggested that reaction may occur in such a 

 way that both potassium and chlorine, for example, may join on to the nitrogen of 

 the NH 2 group, as H and Cl do. Another possibility is that the K may unite 

 with COOH in the usual way, while the Cl joins to the NH.,, with the aid of the 

 H displaced from COOH. These suggestions do not seem very probable from the 

 chemical point of view, although, of course, not impossible. Until recently, no 

 direct evidence had been brought forward in favour of combinations of a chemical 

 kind, but Pfeiffer and Modelski (1912) state that they have obtained crystalline 



