PROTEINS 475 



form of a chain, or thread, which may possibly in certain cases 

 be regarded as a tenuous, crystaUine fiber. This interpretation 

 is based in part on the fact that the proteins are built of amino 

 acids the molecules of which are linear. The synthesis of 

 polypeptides led to four possible structural formulas for glycyl- 

 glycine; of these, Fischer accepted the following linear orienta- 

 tion: NH2— CH2— CO— NH— CH2— COOH. 



Carrying out the idea of a linear structure for the amino acids, 

 polypeptides, and proteins, we obtain the following chain for a 

 protein molecule: 



Ri R3 



.CO. /CH\ /NHv. /CO. /CH. . 



/ \NH/ \C0/ \CH/ \NH/ \C0/ 



I 

 R2 



The — NHCO — union, which occurs recurrently in the above 

 structural formula, is known as the peptide linkage and is regarded 

 as the one most typical of proteins, though not the only one. 

 By means of this and other links, amino acids combine to form 

 polypeptides; polypeptides (with other substances), to form 

 simple proteins; and simple proteins, to form complex proteins. 



While the linear unit is widely recognized as characteristic of 

 proteins, it may unite with other like units to form other figures 

 such as closed rings, or it may itself be a ring. The latter case is 

 illustrated by the "peptine ring": 



I I 



CH— NH— CO— CH— NH— CO 



Larger and more intricate cyclic compounds, or ring structures, 

 are presumed to exist, but the evidence is not conclusive. 



The ring type of protein receives support from the centrifuge 

 experiments of Svedberg, who, from determinations of molecular 

 weight, estimates the diameters of protein molecules, thus 

 suggesting that they are spherical. It may be that the protein 

 thread is coiled into a ball. The consensus of opinion, however, 

 is that the protein molecule is linear. A very possible form 

 which has been suggested is a helix. The helical spiral, while 



