STRUCTURE 



139 



ably indicative of change in molecular shape as environment is changed, and 

 it is in this manner that they are usually used. 



Speed of rotation of a big molecule about an axis can be inferred by an opti- 

 cal measurement calledy/oie» birefringence, and the result related to molecular 

 weight. Both the optical technique involved and discussion of the propor- 

 tionalities are beyond the scope of this outline, for they are very specialized. 



Proper use of the techniques outlined have shown many interesting prop- 

 erties about certain biologically active molecules. Compare now the results 

 of the dynamic methods with those of. static methods. Table 6-2 gives aver- 

 age weight and dimensions of collagen, measured by five different methods, 

 and of erythrocyte DNA by two methods. Our well-worked illustration, Fig- 

 ure 6-6, shows the results of the direct measurements of size of dried 

 ichthyocol** rods by electron microscope techniques as compared with the 

 indirect measurements by light scattering, flow birefringence, and intrinsic 

 viscosity methods. 



TABLE 6-2. Dimensions of Molecules of Collagen and DNA. 



* The chief constituent of connective tissue (cartilage, tendon, etc.). (After Doty, Oncley, etal., Eds. (19 

 "'Extracted from human erythrocytes. (After Butler, el al. (I960).) 



These are particularly pleasing results, one result confirming the other. 

 Such is often not the case for randomly coiled molecules for which the results 

 of different methods may disagree violently with one another. Carbohy- 

 drates are particularly perplexing from this viewpoint. Again, in solution 

 DNA is a very large, randomly coiled molecule, an 'extended double- 

 stranded helix, apt to polymerize and take any shape at all in response to its 

 environment. Therefore the study of nucleic acid reproduction as a molec- 



** As the name implies, ichthyocol is a collagen from fish, used as a food supplement, as 

 are gelatin from animals and glutin from wheat. 



