OPTICAL PROPERTIES OF NUCLEIC ACIDS 539 



to distinguish oriented protein from oriented nucleic acid. The high*"* 

 negative birefringence of extracted high-molecular-weight thymus DNA 

 solution, oriented by flow, was immediately interpreted by Signer, Caspers- 

 son, and Hammarsten*-' as indicating that the absorbing groups were ori- 

 ented with their ring planes perpendicular to the long axis of the macro- 

 molecule, and this conclusion was utilized by Astbury and Bell'" in their 

 model of the DNA molecule in which the nucleotide residues were stacked 

 parallel to each other along one side of the long molecular axis. [Cf. Jordan, 

 Chapter 13.] 



The negative ultraviolet dichroism of a thymus DNA film oriented by 

 stretching was first demonstrated by Caspersson,*^* whose results show a 

 dichroic ratio at the 260-mM absorption maximum of ca. 1.6 and also good 

 agreement between the observed absorption curve for unpolarized light 

 and that calculated from the values obtained with the electric vector paral- 

 lel and perpendicular to the direction of stretch. No dichroism was observed 

 with yeast PNA films, in agreement with indications from other techniques 

 that this material was unoriented. It has usually been considered that 

 because of its lower molecular weight and possibly because of structural 

 differences, PXA will not show molecular orientation to the striking extent 

 found with DNA. However, PNA from calf and rat liver, prepared with 

 precautions against depolymerization,*" shows flow birefringence (about 

 one-tenth of the value for highly polymerized DNA) and forms birefringent 

 fibers which are elastic when undried (cf. sodium DNA fibers, see below). 

 It seems likely that, when the problems of preparing undenatured PNA 

 have been solved, this class of nucleic acids will also lend itself to studies of 

 optical anisotropy. 



Seeds*"* "^'^^^ has obtained dichroic ratios of 1.7-2 for air-dried {ca. 30% 

 water) films of thymus DNA oriented by shearing a viscous gel, the di- 

 chroism being roughly constant over the entire absorption band. Obser- 

 vations over a range of controlled humidity show that the dichroism in- 

 creases with increasing humidity up to 90%, above which the specimen 

 becomes unstable to irradiation and changes eventually to an isotropic 

 form. The highest dichroic ratio recorded by Seeds is 4.7 at the 265-mM 

 maximum for a film in air at 93 % humidity ; the average value for air- 



122a Pq^ oriented fibers with rotational symmetry about the long axis and for their 

 films oriented by shearing, etc.. the sign convention for birefringence and dichro- 

 ism is that a positive fiber has its greater refractive index (or absorbance) along 

 the fiber length. When the greater refractive index (or absorbance) is perpendicu- 

 lar to the fiber axis the sign is negative, as in nucleic acid. (Cf. ref. 101, pp. 148, 441). 



^" R. Signer, T. Caspersson, and E. Hammarsten, Nahire 141, 122 (1938). 



'" T. Caspersson, Chromosoma 1, 605 (1940); cf. J. P. Greenstein, ref. 87. 



1" E. L. Grinnan and W. A. Mosher, J. Biol. Chern. 191, 719 (1951). 



'26 W. E. Seeds and M. H. F. Wilkins, Discussions Faraday Sac. No. 9, 417 (1950). 



