OPTICAL ACTIVITY OF BIOL. MATERIAL 21 



3. Asymmeiry of Primary Constituents of Protoplasm. 

 From the view-point of their asymmetric molecular aggre- 

 gation, the substances which enter into the composition of 

 living systems may be divided into two groups. Physio- 

 logists have for a long time been accustomed to call these 

 two groups, respectively, the primary and the secondary 

 constituents of protoplasm. To the group of primary sub- 

 stances belong the proteins and the lipoids which form 

 together the so-called lipoprotein complexes, and the carbo- 

 hydrates which functionally are closely related to them. 

 These primary substances, except for some stored carbo- 

 hydrates and proteins, build up protoplasm itself and pre- 

 side over the fundamental living processes. To the group 

 of secondary constituents belong various products of trans- 

 formation of the primary substances, which represent 

 either storage material or excreta. 



We shall study, to begin with, the asymmetric structure 

 of primary substances, and we shall consider, first, the 

 degree in which they deviate from the racemic state, or, in 

 other words, their optical purity. 



As far as proteins are concerned, the optical activity of 

 which was already known to Pasteur, Emil Fischer was the 

 first to express the idea that their constituent amino-acids 

 are always found in protoplasm in the optically pure state, 

 and that, when a total or partial racemisation occurs, it is 

 due to the application of too coarse methods of isolation. 

 For instance, serine from silk was known for a long time 

 only in the form of a racemic compound, as it is rather 

 easily racemised in the process of protein hydrolysis. 

 However, Fischer (1907) succeeded in isolating from silk 

 optically active serine, of which the specific rotation (in 

 hydrochloride solution,^ at 18°) was + 11.6°, while the rota- 

 tion of optically pure laevorotatory serine prepared syn- 

 thetically {i.e., crystallized from racemate with alkaloids) 

 was + 14.4° (at 20°). On the ground of these experimental 

 data alone it is, of course, impossible to conclude that serine 

 in silk is optically pure, and that it is partially racemised 



1 The hvdroc'liloride of laevorotatory serine is dextrorotatory. 



