OPTICAL PROPERTIES OF NUCLEIC ACIDS 535 



ner^"* are isotropic rods arranged in parallel bundles and flat plates in paral- 

 lel stacks, separated in both cases by an amorphous component of different 

 refractive index. Wiener's equations apply strictly to systems in which 

 the volume concentration of the ordered phase is small and is therefore 

 not accurate for, e.g., a slightly swollen fiber, in which the ordered phase is 

 a large proportion of the total volume. It has been stated^°^ that a more 

 accurate theory for hexagonally packed rods is being prepared. Examples 

 of the use of the Wiener formulas have been given by many authors'"^"^"^ 

 in connection with the optical properties of natural cellulosic fibrous struc- 

 tures. 



Form anisotropy can be detected by changing the refractive index of 

 the amorphous component (by immersion in liquids of various refractive 

 indices) and observing the effect on the birefringence. At a particular re- 

 fractive index of the immersion li(iuid the form birefringence (positive for 

 rod model, negative for lamellar model) falls to zero, and any residual 

 birefringence is then intrinsic, due to anisotropy of the macromolecules in 

 the ordered phase. 



A good example of this behavior was given by Weber"" (quoted by Doty 

 and Geiduschek'") for artificial gelatin and myosin fibers; both showed 

 positive l)irefringence, but, at the critical refractive index, the residual 

 birefringence of the gelatin fiber was zero, while for the myosin fiber a large 

 intrinsic birefringence persisted. Some observations on nucleoproteins and 

 nucleic acids in which form anisotropy may have been involved will be 

 noted later in this section. 



3. Definitions and Equations'"' 



A linear absorbing system is characterized by two absorptivities, ay and 

 a± for the electric vector parallel and perpendicular, respectively, to the 

 direction of the absorbing system. Then 



Dichroism = an — a± 



Dichroic Ratio (D) = a\\/ax 



In general a biaxial crystal will have three absorbances (Ax , A„ , A^) 

 corresponding to the principal orthogonal axes of the triaxial absorption 



'"« O. Wiener, Abhandl. math.-phys. Kl. sacks. Akad. Wiss. (Leipzig) 32, 507 (1912)-, 

 KoUoidchem. Beih. 23, 189, 198 (1926). 



»»» M. H. F. Wilkins, A. R. Stokes, W. E. Seeds, and G. Oster, Nature 166, 127 (1950). 



I" F. O. Schmidt, Advances in Protein Chem. 1, 25-68 (1944). 



'"* A. Frey-Wyssling, "Submicroscopic Morphology of Protoplasm," Elsevier, Ams- 

 terdam, London, New York, Houston, 2nd English Ed., 1953. 



»»» A. Frey, Jahrb. wiss. Botan. 67, 597 (1927). 



"« H. H. "Weber, Pfliigers Arch. ges. Physiol. 235, 205 (1934). 



'» P. Doty and E. P. Geiduschek, in "The Proteins" (H. Neurath and K. Bailey, 

 eds:). Vol. 1, part A. Academic Press, New York, 1953. 



