Proteins and Protoplasmic Structure 35 



Mirsky found it possible to isolate the labile protein fraction by 

 salting out the soluble protein from unfertilized eggs with ammonium 

 sulfate, whereupon the labile fraction came down first. It was then 

 redissolved. The physical properties of this protein in its soluble 

 state are precisely what might have been expected from the fore- 

 going discussion of protoplasmic properties. At pH 7, it appears to be 

 highly elongated. The viscosity of a 1.4 per cent solution was 9.6 

 times that of water. This value was influenced by the rate of shear, 

 so that solutions of the protein exhibit structural viscosity. Unlike 

 most of the corpuscular proteins, it showed streaming double 

 refraction.^ 



Myosin is very like this protein in many respects. Its solutions 

 show streaming double refraction and structural viscosity. They 

 can readily be brought into a thixotropic state and exhibit marked 

 elasticity (9, 25) . When muscle is brought into rigor, its myosin 

 becomes insoluble (25) . It thus appears that the physical properties 

 of two proteins associated with protoplasmic structure are strikinglj^ 

 similar. 



Physical State of These Proteins. Changes induced in the cor- 

 puscular proteins by heat, acid, etc., usually involve two steps: 

 (1) denaturation and (2) coagulation. In denaturing, the molecule 

 unfolds and the viscosity of the solution is increased. The mechanism 

 of this change is not clearly understood, but it is usually agreed 

 (26, 35) that weak bonds, such as hydrogen bonds, holding the mole- 

 cule together, are broken and the molecule becomes an extended 

 structure, approximately 10 A thick. The denatured product is not 

 rendered insoluble until the pH of the solution is brought into the 

 neighborhood of its isoelectric point, whereupon the protein coagu- 

 lates and loses its solubility in water (6) . This change is presumably 

 the result of a side-by-side association after the net charge is de- 

 creased. In most cases, denaturation of these proteins is accompanied 

 by a change in the number of sulfhydryl groups (25, 26) . 



The fibrous proteins, on the other hand, are considered to be 

 already unfolded, i. e., denatured and coagulated. The X-ray inves- 

 tigations of Astbury (3) indicate that this unfolding varies in extent. 

 Astbury believes that keratin, as it exists in unstretched mammalian 

 hair, is in what he calls the a state. On stretching the hair, the 

 backbones become completely extended (|3 keratin) ; if, however, 

 relaxed hair is treated with steam, they become "supercontracted." 

 These states may not be produced by such simple foldings of the back- 



