34 PHYSIOLOGY. 



In each, the first and last groups are the same, the middle group 

 varies and may be represented by E. The general formula of the 

 mono-amino acids is, therefore: HNH, E, COOH. 



If .now we link these two together, we get HNH, E, CO, 



|OHH NH, E, COOH. What happens is that the hydroxyl 



(OH) of the carboxyl (COOH) group of one acid unites with one 

 atom of the hydrogen in the next amino (HNH) group, and water 

 is thus formed, as shown in the oblong, and the rest of the chain 

 closes up and the water is eliminated. In this way we get a dipep- 

 tid. The names glycyl, alanyl, leucyl, etc., are given by Fischer to 

 the NH 2 , E, CO groups which replace the hydrogen atom of the 

 next NH 2 group. 1 



Thus: glycyl-glycin, glycyl-leucin, leucyl-alanin, alanyl-leucin, 

 and numerous other combinations and permutations are obtained. 



If the same operation is repeated, we obtain tripeptids (leucyl- 

 glycyl-alanin, alanyl-leucyl-tyrosin, etc.); then we have the tetra- 

 peptids and so on; and in the end, by coupling the chains suffi- 

 ciently often and in appropriate order, Fischer has already obtained 

 substances which give the reaction of a peptone. 



Hence we may consider proteids as essentially polypeptid com- 

 pounds of greater or less complexity; that is, they are acid-amids 

 formed by the union of a number of amido-acid compounds. Many 

 of the polypeptids have been produced synthetically, and these facts 

 lead to the hope that the actual synthesis of the proteid molecule 

 may be finally accomplished. 



About 100 polypeptids have been synthetically prepared. The 

 variety of forms possible is almost infinite when one considers the 

 variety of combinations which may take place. The peptids and 

 polypeptids ' synthetically prepared are not split into their com- 

 ponent amido-acids by pepsin, but trypsin splits some of them, 

 especially those formed from the optically active amino-acids which 

 occur in nature. 



The inquiry into polypeptids of a definite construction has led 

 to the isolation of such compounds directly from the proteins, and 

 will aid us in understanding the more complex substances, as pro- 

 teoses and peptones. They consist of mixtures of complex polypeptids, 

 but at present our methods are not perfect enough for separating poly- 

 peptids from one another and from amino-acids. 



British Medical Journal. 



