14 CHEMICAL STATICS 



substitution of phenylalanin, tyrosin and cystin in the place of 

 glycin groups would increase this weight two or three times, 

 giving a value which is of the same order as that of the more 

 modern estimations of the (minimal) molecular weights of many 

 of the natural proteins.* 



A whole series of the polypeptids give the typical peptone 

 biuret reaction, and such as contain tyrosin give also Millon's 

 reaction. The biuret reaction is, with the glycin compounds, 

 first encountered in the tetrapeptid, but it is given by other 

 tripeptids. It is more intense the greater the length of the poly- 

 peptid chain, and it is also intensified by the carboxyl group or 

 by conversion of the carboxyl group into an acid amide group. 

 The majority of the polypeptids are readily soluble in water, 

 and such as are with difficulty soluble in water are readily soluble 

 in dilute mineral acids and alkalies with which they combine; 

 they are less soluble in solutions of acetic acid. As a rule they 

 are insoluble in absolute alcohol, but in alcohol containing a 

 little watery ammonia they are generally soluble; on boiling off 

 the ammonia they are precipitated again. Under conditions 

 involving dehydration, e.g., heating or treatment of the esters 

 with alcoholic ammonia, the dipeptids are converted into diketo- 

 piperazines which are ring-compounds. Under similar conditions 

 the polypeptids are modified in an analogous manner, with the 

 formation of ring-compounds. 



Upon hydrolysis the peptids break down into their constituent 

 amino-acids, the imino groups in the polypeptid molecule being 

 converted, by the taking up of water, into amino groups. A 



* Emil Fischer strongly inclines to the opinion that the higher estimates, 

 such as 12,000 and 15,000 for the minimal weight of the protein molecule, 

 which are freely cited in the older literature upon proteins, are in error, since 

 the admixture of a small quantity of another protein might easily raise the 

 calculated value to this magnitude, and we have no proof of the chemical 

 individuality of the majority of our protein preparations, even when, as some- 

 times happens, they are crystallizable. An exception appears to be afforded 

 by haemoglobin, the minimal weight of which is placed by many observers, 

 employing adequate chemical and physico-chemical technique, at or in the 

 neighborhood of 16,000. But then haemoglobin is probably to be regarded as 

 a salt-like compound of, possibly, two or more molecules of a basic, histone- 

 like protein (globin) with a non-protein acid, namely hsematin. In a similar 

 way, we shall see (Chaps. V and IX) that casein, which is possessed of a 

 minimal molecular weight of 4000-4400, forms, under certain conditions, salts 

 of which the molecular weight is double or four times this. 



