102 PRINCIPLES OF GENERAL PHYSIOLOGY 



PROTEINS 



In many respects proteins are the most important members of the emulsoid 

 class and, at the same time, the most difficult to treat in a satisfactory way. This 

 difficulty is mainly due to the fact that the phenomena presented by them can be, 

 for the most part, described from two different points of view, from that of pure 

 structural chemistry and from the physico-chemical standpoint of colloidal 

 chemistry. 



Take, for example, the common test for the presence of a protein in solution, that 

 with potassium ferrocyanide and acetic acid. Potassium ferrocyanide alone, in low 

 concentration, does not give a precipitate, hut such appears when the solution is ni;u It- 

 acid with acetic acid. This may be explained by saying that the compound of protein 

 with ferrocj'anide is soluble in neutral or alkaline solution, insoluble in acid. Or by 

 saying that the negative ferrocyanic ion has no precipitating action on an electro-negative 

 colloid, as protein is in neutral or alkaline solution, but becomes a powerful one, as 

 a quadrivalent ion, when the colloid is made positive by the H' ion of an acid. 



Now these two points of view are not to be regarded as antagonistic or 

 mutually exclusive. As Perrin remarks (1905, p. 110), with respect to the fact 

 that change of electrification is accompanied by change in the composition of the 

 double layer and, therefore, in the composition of the colloid as given by chemical 

 analysis, " physical and chemical variations are here two aspects of one and the 

 same phenomenon." 



On the other hand, there are certain physical properties not necessarily 

 involved in the chemical description of proteins, which must play a part in their 

 behaviour. Since they do not diffuse through parchment paper, we know that 

 they are large enough to exhibit the properties of matter in mass, the most 

 characteristic being those connected with the possession of surface. This involves 

 electrical relations differing from those of simple electrolytes, and so forth. 



When we find a book, " The Physical Chemistry of the Proteins," by J. Brailsford Robertson, 

 1912, which professes to treat the whole subject without reference to any of the conceptions 

 which the modern development of the theory of the colloidal state has introduced, wi- 

 cannot but agree with the reviewer (W. O.) in Zeitsch. f. physik. Chemie, 81, 508, whose 

 remarks will serve to call attention to the facts to be taken into consideration in an 

 adequate treatment. "But in so far as the colloidal, that is non- homogeneous, charm t< i 

 of protein compounds has been proved experimentally beyond all douot, it appears to 

 me (the reviewer) that, in the intentional laying aside of this fact, an error of method 

 is committed, an error which brings with it considerable danger of laying more weight 

 on a particular interpretation of facts, in themselves correctly observed, than is desirable 

 in the interests of science. This danger is all the more serious when the author is one 

 who is able to manipulate, with considerable skill and corresponding predilection, complex 

 mathematical formulae, and by that means is able to introduce as many variables into 

 the theoretical treatment of his problems as satisfactory agreement with experimental 

 results requires. Owing to the complex and changeable nature of the substances in 

 question, a completely exact agreement between measurement and calculation can never 

 be expected, and, for this reason, the widest possibilities for theoretical presentation 

 offer themselves." 



With reference to the two theories, the purely electro-chemical and the colloidal, 

 where the conditions at finite boundary surfaces are taken into account; the same reviewer 

 says, " The two are, in point of fact, nowhere and never in opposition, but are merely 

 different stages in the complete analysis of the physico-chemical phenomena." 



An illustration due to A. W. Stewart (Chemical World, 2, 53) will serve to show the 

 misleading effect of a one-sided consideration. "Suppose that we gave two specimens, a 

 diamond and some graphite, to be examined by purely chemical methods on the one hand, and 

 by purely physical methods on the other. The chemist, relying on his analysis, would declare 

 them to be identical, consisting, as they do, of carbon. The physicist, on the other hand, 

 from an examination of colour, density, form, etc., would pronounce them to be different from 

 one another. Both would be right, but each in possession of half the truth only." If we 

 wanted to cut glass, it would not be of any help to be told that the chemical composition of 

 diamond and graphite is the same. On the other hand, suppose that we wanted to prepare 

 carbon dioxide by burning in oxygen, either would serve, although we should hardly choose 

 the diamond for the purpose. Should the electronic theory of the constitution of atoms be 

 correct, reconciliation between the chemical and physical points of view will presumably be 

 found in molecular physics. 



In order to indicate the kind of problems which confront us in the study of 

 proteins, the work of Hopkins and Savory (1911, p. 249) on Bence-Jones' 

 protein, may be referred to. This substance is found in the urine in certain 

 disorders of metabolism, and is characterised by its peculiar behaviour on heating. 



