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



band frequencies and assignments proposed by this author are in substantial agree- 

 ment with those given by Fraser;'*" the 960-cm.~' band is greatly reduced in intensity 

 by enzymic degradation and is therefore correlated with the internucleotide ester 

 linkage (P — O — Cs'), the 1015-cmr' band then being regarded as arising in the nucle- 

 oside-phosphate bond (P — O — Cs')- The assignments of vibrations involving the 

 phosphate group are very important for the interpretation of the infrared absorption 

 and dichroism of nucleic acids and are briefly discussed by Fraser.^'" 



The dichroism of infrared absorption bands has been used in recent structural 

 studies on nucleic acids. The theoretical basis of the method is the assumption that 

 the transition moment associated with a vibration which is active in absorption is 

 either along or perpendicular to the band involved, depending on whether the vibra- 

 tion is bond-stretching or bond-bending, respectively. The dichroism of the C=0 

 stretching and N — H bending bands has been widely used to investigate the a- and 

 /3-configurations of oriented structural proteins and synthetic polypeptides (for 

 literature references see Fraser'^" and Short and Thompson'®^) Fraser and Price'*' 

 have shown, however, that in the important case of the peptide linkage this simple 

 assumption may not be justifiable. A consideration of the mechanical interactions 

 involving atoms other than the bonded pair in the C=0 group, and of the effect of 

 the vibration on the resonance structures of the peptide group, indicate that the di- 

 rection of the transition moment for the 0=^0 stretching vibration will not be exactly 

 along the bond. The interpretation of infrared dichroism may therefore have to be 

 made with considerable caution. 



Fraser and Fraser'** have studied the infrared absorption and dichroism of shear- 

 oriented films of thymus DNA and assigned the principal bands in accordance with 

 the general classification given above. The N — H stretching vibrations of the base 

 amino and imino-groups and the double-bond (C==0, C=N, C=C) stretching vibra- 

 tions of the same ring systems show perpendicular dichroism, i.e., absorption is 

 greatest when the electric vector is perpendicular to the direction of shear, which is 

 taken as the direction of the polynucleotide chain. In agreement with the negative 

 birefringence and ultraviolet dichroism, the perpendicular infrared dichroism indi- 

 cates that the planar bases are approximately perpendicular to the chain axis. The 

 dichroism of other bands which are associated with the sugar residue and the phos- 

 phate group also appears to be consistent with the modifications proposed by Fur- 

 berg'" to the Astbury polynucleotide model. [Cf. Jordan, Chapter 13.] Fraser and 

 Fraser have also found that the type A — > type B transformation of DNA which 

 occurs on stretching at a suitable humidity (cf . Section IV) is accompanied by a re- 

 versal from perpendicular to parallel of the dichroism of manj^ important bands; the 

 effect is most marked with the 967-cm.~' bond, which has not yet been assigned with 

 certainty. The bond at 1235 cm."' with a dichroic ratio of unity, is not affected by 

 this structural transformation. The assignment of this bond to a P==() stretching 

 vibration'*' has been reconsidered.'** 



Fraser"* has studied the infrared dichroism of shear-oriented tobacco mosaic virus 

 gels and finds the frequencies and dichroism of theC=Ostretchingand N — H bending 

 vibration bonds to be consistent with a structure in which the protein is in an a-con- 



'" R. D. B. Fraser and W. C. Price, Nature 170, 490 (1952); see also A. Elliott, ihiil. 



172, 359 (1953). 

 '** M. J. Fraser and R. D. B. Fraser, Nature 167, 761 (1951). 



'*^ S. Furberg, Acta Chem. Scand. 4, 751 (1950) ; see D. O. Jordan, ref . 6, for summary. 

 "* R. D. B. Fraser, Nature 170, 490 (1952). 



