540 



G. H. BEAVEN, E. R. HOLIDAY, AND E. A, JOHNSON 



1 1 1 \ 1 



— Electric vector perpendicular to direction of shear 

 • — Electric vector parallel to direction of shear 



240 



250 



290 



260 270 280 



Wavelength, rriM 

 Fig. 27. Ultraviolet absorption spectra of oriented film of thymus DNA sodium 

 salt: (+) at 90% humidity, (O) at 60% humidity (Seeds 'o^, ii6). 



dried oriented DNA is ca. 2, increasing to nearly 4 at 90 % humidity, (Fig. 

 27.) The change in dichroic ratio with humidity can be interpreted, using 

 the equations for model systems as a rotation of the planes of the absorbing 

 groups, the angle obtained depending on whether the uniaxial fiber or type 

 II sheet model is selected. Seeds concludes that the bases are so arranged 

 that, on the average, they lie at a small angle to the normal to the mo- 

 lecular axis and that this angle decreases with increasing humidity; the 

 actual structure is probably intermediate between the two model structures 

 considered. 



The influence of humidity on the anisotropy of oriented DNA fibers has 

 been studied in greater detail by Wilkins, Gosling, and Seeds,^" who rec- 



1" M. H. F. Wilkins, R. G. Gosling, and W. E. Seeds, Nature 167, 759 (1951). 



