Proteins and Protoplasmic Structure 29 



occurred, X-ray photographs show crystal patterns similar to those 

 yielded by protein fibers. 



Between the corpuscular and truly fibrous proteins are such 

 proteins as the muscle protein, myosin, and tobacco mosaic virus. 

 These, although so elongated that they exhibit streaming double 

 refraction, are not completely extended and disperse readily in 



asporfic add lysine 



COOH NHp 



I J \ 



l"*^ H T^'^H 

 N C C N C C N 



/ W \ A Ah/ \ /H\ / \ 



C N C C N C 

 I HO I H 

 CH2 



leucine tyrosine 



Fig. 2. Portion of a hypothetical protein molecule showing typical side 

 chains. 



aqueous solutions (9). The lability of proteins of this type has 

 probably retarded the discovery of many similar examples. 



PHYSICAL CHARACTERISTICS COMMON TO PROTEINS AND 



PROTOPLASM 



Gel Formation. One of the properties of protoplasm is its capacity 

 to exist as a gel. It is a striking characteristic of many of the more 

 extended proteins that these too can form gels at low concentrations. 

 Staudinger (46) has emphasized that it is not enough for a particle 

 to be rod-shaped; to form a gel it must be able to form cross- 

 linkages and a net structure. Chains of polystyrene are dispersed by 

 organic solvents. Staudinger found that, if as little as 0.002 per cent 



