PROTEINS 165 



Protein molecules are very large and, in the case of the so- 

 called " conjugated proteins " in particular, their structure is very 

 complex. The molecular weight of some of the proteins has been 

 determined directly, in the case of those particular ones which can 

 be prepared in proper form for the usual determination of molecular 

 weight by the osmotic pressure method; and has been computed 

 for various others, from the percentage of sulfur found on analysis, 

 or (in the case of the haemoglobin of the blood) from the propor- 

 tion by weight of oxygen absorbed. From these determinations 

 and computations, the following formulas for certain typical 

 proteins have been calculated: for zein (from Indian corn), 

 C736Hn6iNi84O208S3; for gliadin (from wheat), CessHioGsNige- 

 021185; for casein (from milk), C708Hn3oNi8oO224S4P4; for egg- 

 albumin, C69oHn25Ni75O22oS8. These few examples will serve 

 to illustrate the enormous size and complexity of the protein 

 molecule. The conjugated proteins are still more complex than 

 the simple proteins whose formulas are here presented. 



Fortunately for the purposes of the study of the chemistry of 

 the proteins, however, it has been found that most of the common 

 plant proteins, known as the " simple proteins," can easily be 

 hydrolyzed into their constituent unit groups, which are the com- 

 paratively simple amino-acids, whose composition and properties 

 are well understood. A study of the results of the hydrolysis of 

 some twenty common plant proteins has shown that it is rarely 

 possible to recover the amino-acids in sufficient quantities to 

 account for a full 100 per cent of the material used, the actual 

 percentage of amino-acids recovered usually totaling from 60 to 

 80 per cent. The remaining material is supposed to be also com- 

 posed of amino-acids which are linked together in some arrange- 

 ment which is not broken apart by any method of hydrolysis 

 which has yet been devised. This view is borne out by the fact 

 that substances which exhibit all the characteristic properties of 

 proteins have been artificially synthetized, by using only amino- 

 acid compounds. Animal proteins often show a much larger pro- 

 portion of unhydrolyzable material than do plant proteins. 



