PROTEINS 489 



found that the white of an egg diluted with distilled water, 

 filtered, and boiled migrates in an electric field — in one direction 

 when the solution is acid and in the opposite direction when 

 alkaline. Obviously, when the solution is neutral, there should 

 be no migration. This is the isoelectric point. Neutrality here 

 means electrical and not acid-alkaline neutrality, though the 

 isoelectric point is most often expressed in terms of pH. The 

 isoelectric point is the point of zero potential (or nearly so) of a 

 colloidal particle, and it is usually not at acid-alkaline neutrality, 

 i.e., at pH 7. For proteins, it is apparently always, certainly 

 most often, on the acid side; in other systems, it may be on the 

 alkaline side (rarely, there are two isoelectric points, one acid 

 and one alkaline). The isoelectric point of gelatin is at pH 4.7. 

 Proteins as Colloidal and Crystalloidal Systems. — There can 

 be no doubt as to the colloidal nature of proteins; their capacity 

 to form gels, to swell in water, and to exhibit the Tyndall phe- 

 nomenon is typical colloidal. But it may be that under certain 

 conditions they lack one of the most characteristic properties of 

 colloids, viz., the micelle. While this lack, if it exists, may from 

 one point of view be of no great consequence — for the protein 

 molecule is itself sufficiently large to account for colloidal prop- 

 erties — yet there is another side to the question. If proteins 

 are molecularly dispersed when in solution, they should behave 

 like a salt, as we have assumed that they do. If they are 

 colloidally dispersed, they should show those colloidal properties 

 characteristic of surfaces, such as adsorption. Let us consider 

 a simple case. If hydrogen ions disappear from a solution of 

 hydrochloric acid in which gelatin is immersed, then these ions 

 may either have united in stoichiometrical proportion with the 

 gelatin to form a salt (gelatin chloride), all according to the laws 

 of valence, or the hydrogen may have been selectively adsorbed 

 by the gelatin particles according to a colloidal law, such as 

 Freundlich's adsorption isotherm, which does not follow the law 

 of valence. The assumption that proteins form salts with acids 

 and bases is too well supported by experimental facts to deny its 

 possibility. On the other hand, pure adsorption phenomena 

 have also been so well established that the colloidal viewpoint 

 of protein reactions appears to be definitely settled. It is quite 

 likely that both colloidal and molecular reactions occur simul- 

 taneously in protein solutions and that one or the other dominates 



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