70 PHYSIOLOGICAL CHEMISTRY 



invariably acids, as has already been mentioned, and contain an NH2 

 group in the a position. This relation of the NH 2 group to the acid radi- 

 cal is constant, no matter what other groups or radicals are present. We 

 may have straight chains as in alanine and glutamic acid, the benzene 

 ring as in phenylalanine, or we may have sulphurized bodies as in cystine 

 and still the formula is always of the same type, i.e., 



NH 2 



I 

 R CH COOH 



It is seen that this characteristic grouping in the amino acid provides 

 each one of these ultimate fragments of the protein molecule with both a 

 strong acid and a strong basic group. For this reason it is theoretically 

 possible for a large number of these amino acids to combine and the re- 

 sulting combinations may be very great in number, since there is such 

 a varied assortment of the acids. The protein molecule, which is of 

 such mammoth proportions, is probably constructed on a foundation of 

 this sort. Much valuable data have been collected regarding the syn- 

 thetic production of protein substances, the leaders in this line of in- 

 vestigation being Fischer and Abderhalden. After having gathered a 

 mass of data regarding the final products of the protein decomposition 

 and demonstrating that amino acids were the ultimate results of the 

 various forms of decomposition, these investigators, and notably 

 Fischer, set about in an effort to form, from these amino acids, by 

 synthetic means, substances which should possess protein char~acter- 

 istics. The simplest of these bodies formed in this way was synthesized 

 from two molecules of glycocoll with the liberation of water, thus : 



CH 2 -(NH 2 )-CO OH H HN-CH 2 -COOH. 



The body thus formed is a dipeptide, called glycyl-glycine. In an analo- 

 gous manner may be produced leucyl-leucine, through the synthesis of 

 two molecules of leucine or leucyl-alanyl-glycine through the union of one 

 molecule of leucine, one of alanine, and one of glycocoll. By this pro- 

 cedure Fischer and his pupils have been able to make a large number of 

 peptides containing varied numbers of amino acid radicals, the name 

 polypeptides being given to the whole group of synthetic substances thus 

 formed. One of the most complex polypeptides yet produced is one 

 containing fifteen glycocoll and three leucine residues. 



Notwithstanding the fact that most synthetic polypeptides are pro- 

 duced through a union of amino acids by means of their imide bonds, it 

 must not be imagined that the protein molecule is constructed from 



