4 STUDIES IN GELS 93 



The birefringence of flow has furnished arguments in favour of the 

 view that the micellar strands of protein gels are beaded chains (Fig. 51a, 

 p. 66). A flowing solution of 1.5% gelatin is isotropic at 40° C. This is an 

 indication that this sol contains globular protein molecules. When cooled 

 down to 20° C. the gelatin sets after some time. During the incipient 

 gelification the solution becomes birefringent owing to the formation of 

 micellar strands. The extinction angle of the double refraction of flow 

 permits calculation of the length of the elongated particles in such a gel 

 solution. Whereas a diameter of only about 50 A must be attributed to the 

 globular protein molecules, the measured chain length is more than 1000 A 

 and it increases steadily up to over 6000 A before the system solidifies. 

 Particles of this length could not possibly be formed by unfolding of the 

 polypeptide chain, which is somehow coiled in globular protein molecules ; 

 its cross-section measuring about 46 (A)^ (see p. 365), its length could not 

 exceed 1500 A when coiled in a sphere of 50 A diameter. Joly (1949) 

 therefore concludes that the micellar strands result from linear aggregation 

 of globular macromolecules forming beaded chains. When these have 

 become sufficiently long, they interact and a three dimensional network, 

 i.e. a gel, is formed. This gel, containing 1.5% gelatin by volume and 

 micellar strands of 50 A diameter, must have a relatively wide-meshed 

 network. Assuming that the beaded chains meet with the tetrahedron angle 

 of 109.5°, the edges of the polyhedra which compose the framework are 

 as much as o.i /x long. 



The force of aggregation in these gelatin chains is weak. By increasing 

 the velocity gradient in the apparatus inducing birefringence of flow, the 

 micellar chains of a gel solution of gelatin are shortened by rupture. The 

 applied force couple is of the order of Van der Waals forces, an indication 

 that no valency bonds have been formed between the beads of the chain. 

 This is the reason why a gelatin gel can be melted and reduced to a sol by 

 simple heating. 



According to Joly (1949), the same beaded chains are formed when 

 proteins with globular molecules are denatured (seep. 136), e.g. when a 

 solution of blood albumin is heated. At a certain temperature intra- 

 molecular bonds are loosened and become free to replace the Van der 

 Waals bonds between the aggregated molecules by chemical bonds, such 

 as hydrogen-, salt- or ester-bonds (see p. 145). Then the protein has become 

 insoluble and, therefore, denatured. 



Similar observations have been reported of ovalbumin by Foster 

 and Samsa (1950). This protein consists of relatively small globules 

 (Fig. 2, p. 11) which can be unfolded by a high flow gradient to 

 sinuous chains of 600 A length. But this occurs only when the con- 

 centrations are low (< 0.6%). Particles of 2000 A length have been 

 measured in more highly concentrated solutions (2.4%). Such lengths 



