OPTICAL PROPERTIES OF NUCLEIC ACIDS 541 



ognize two distinct forms with strikingly different properties: 

 Type I (stable at high humidity) with : 



Negative birefringence 



Negative ultraviolet dichroism 



Mainly negative infrared dichroism 



X-ray diffraction pattern of well-ordered crystallites 

 Type II (stable at 50% humidity) with: 



Positive birefringence 



No ultraviolet dichroism 



Mainly positive infrared dichroism 



Diffuse-X-raj'- diffraction pattern 

 Type II can be obtained from a type I fiber by stretching up to nearly 

 100 % elongation in air of 50 % relative humidity. Reconversion to type I, 

 with appreciable contraction, occurs on exposure to higher humidity. 

 Wilkins ct al. consider that the dimensional change associated with hy- 

 dration is due to conversion from amorphous to the crystalline state and not 

 to water molecules packing between the crystallites. On dehydration it is 

 suggested that the molecular backbone of phosphate ester linkages crumples 

 and the bases tilt with consequent destruction of the ultraviolet dichroism 

 (and reversal of birefringence), while in the hydrated I-form the extended 

 molecules pack together regularly with the planes of the bases approx- 

 imately at right angles to the long axis; in the Il-form the rings are estimated 

 to lie at an average angle of ca. 45° to the fiber length. Additional infor- 

 mation on the difference between the I- and II- forms of DNA is given by 

 Franklin and Gosling^-^ in a discussion of X-ray diffraction evidence sup- 

 porting the Watson and Crick" double coaxial helical structure for sodium- 

 DNA. They consider that in the highly ordered I-form at 75% humidity 

 the planes of the base rings are at an angle of ca. 25° to the fiber axis; this 

 value, and that estimated by Wilkins ct al.™ for the Il-form, appear to be 

 consistent with the changes in dichroism observed for the I :^ II intercon- 

 version process. 



It is evident that in these structural studies the ultraviolet dichroism 

 has provided valuable supplementary information to the X-ray diffraction 

 and other optical investigations. [Cf. Jordan, Chapter 13.] 



Work has been reported on the ultraviolet dichroism of the nucleoprotein 

 (containing PNA) associated with tobacco mosaic virus. Butenandt and 

 co-workers^-' measured the dichroism of solutions oriented by streaming, 

 and their essential results have been confirmed by Seeds and Wilkins^^^"^ '^-^ 

 working with both flow-oriented solutions and shear-oriented gels, who find, 



128 R. E. Franklin and R. G. Gosling, Nature 171, 740 (1953); 172, 156 (1953). 



129 A. Butenandt, H. Friedrich-Freksa, St. Hartwig, and G. Scheibe, Z. -physiol. 

 Chem. 274, 276 (1942). 



