6 THE GENERAL CHARACTERS OF THE PROTEINS 



difficulty in obtaining reliable physical constants for proteins, for the 

 physical differences between free bases and free acids, and the salts 

 obtainable therefrom, are, in the case of almost all physical constants, 

 very considerable. To indicate but one instance : Leucine as a 



r ~~]2o 

 free base has an optical rotation of a = - 10*42, whereas the 



rotation of its hydrochloride is'.j&j = + 15*33 in 2O P er cent. 



hydrochloric acid. 



Electrolytic conductivity, osmotic pressure, and many other 

 physical properties are also profoundly affected by the above con- 

 siderations, and attention has been called in the following pages to 

 the circumstances which so often render physical determinations 

 unreliable. 



There is still one other disturbing influence to be noted. Owing 

 to the amphoteric character of the proteins, and their capacity to form 

 salts with acids and bases, they can also conceivably form compounds 

 with neutral salts. Now colloids have a considerable power of ad- 

 sorbing other bodies, a circumstance which also, as already mentioned, 

 considerably affects the determinations of the acidic and basic func- 

 tions of a protein. It is seldom, if ever, that an ash-free protein 

 is obtainable, and it is impossible to determine whether the ash 

 represents inorganic substance in actual combination, e.g.^ sodium in 

 the form of a sodium salt, or whether it owes its origin to bodies 

 which have been physically adsorbed ; the mineral substances present 

 have, under any circumstances, a great influence on the physical 

 properties of the protein. 



The above considerations render the majority of the physical 

 properties extremely unreliable for the characterisation of the 

 proteins ; the precipitability by salts is, alone amongst the properties 

 generally determined, but slightly affected by them. 



Chemical Characteristics. 



It is upon the chemical properties, therefore, that reliance must 

 be chiefly placed for obtaining constants for the characterisation of 

 proteins. 



The most reliable of these are undoubtedly the numbers represent- 

 ing the distribution of nitrogen in the molecule, and generally known 

 as the " Hausmann numbers ". These indicate the relative propor- 

 tions of nitrogen in the molecule combined in the form of amide, 

 of monoamino acids, and of basic bodies. 1 They are discussed in 

 detail on pp. 33-36. 



The number of " active " 2 amino groups also varies in the 

 different proteins ; to determine these Dr. Horace Brown has 

 suggested the determination of the " amino-index " (p. 67). Another 

 suggestion for the determination of this factor is due to Messrs. 

 Cross, Bevan and Briggs (p. 65). The capacity also for forming 

 halogen derivatives varies considerably in the different proteins ; the 

 halogen numbers should serve also as a method of characterisation. 

 Furthermore, different proteins yield different quantities of hydrolysis 



1 See p. 4. 2 Footnote, p. 4. 



