OPTICAL PROPERTIES OF NUCLEIC ACIDS 551 



figuration with the polypeptide chains perpendicular to the axis of the rod-like virus 

 particle, unlike some fibrous proteins which have been studied by the same method. 

 Ambrose and Butler'^' have determined the dichroism in the 1500-1800-cm.~' 

 region of herring sperm nucleoprotein oriented by being cast on a rubber film base 

 which may be stretched to produce about 100% extension. The dichroism of the 

 1695-cm.~' stretching vibrations of the base C=0 and C==N bonds are consistent 

 with the dichroism found bj' Fraser and Fraser'^* for the moderately extended and 

 optically negative type A nucleic acid fiber. The protein, however, is in the |3-con- 

 figuration in this nucleoprotein. There seem to be interactions between the two com- 

 ponents of a nucleoprotein system which determine, on the one hand, the configura- 

 tion assured by the protein and, on the other hand, the internal order of the poly- 

 nucleotide chain. 



It can be seen from this brief account of previous work that there are 

 many practical and theoretical difficulties associated with the infrared 

 absorption spectroscopy of nucleic acids. Further progress in the assign- 

 ment of frequencies and in resolving uncertainties in the directions of transi- 

 tion moments should lead to significant contributions to knowledge of the 

 detailed structure of nucleic acids. 



Addendum 



Bases, Nucleosides, and Nucleotides 



Fox et al."^'' report that the ultraviolet absorption spectra of cytidylic 

 acids a and b behave in a significantly different manner in the alkaline pH 

 range, and attribute this to the presence or absence of an ionizable 2'- 

 hydroxyl group. The spectrum of cytidylic acid a, like that of 2'-deoxy- 

 cytidine, remains constant in the pH 12 to 14 region, and hence may be 

 presumed to be the 2'-phosphate ester, in which no 2'-hydroxyl group 

 ionization can occur. This agrees with other evidence. The specific effect of 

 2'-hydroxyl ionization in the b or 3'-isomer may be due to hydrogen bonding 

 between this group and the 2-keto group of the pyrimidine nucleus, since 

 in the 2'- and 3'-adenylic acids, where such hydrogen-bonding cannot occur, 

 no changes in the alkaline pH region are observed with either isomer. 

 Spectroscopic constants for the isomeric cytidylic acids have also been re- 

 ported by Harris ef al.^^^ and by Cavalieri"^''. 



E. L. Bennett^" and L. L. Bennett"^ have synthesized a number of C^^- 

 containing purines and pyrimidines, respectively, and report brief spectro- 

 scopic data which are in good agreement with values for the normal com- 



'" E. J. Ambrose and J. A. V. Butler, The physical chemistry of proteins, Disc^issions 



Faraday Soc. No. 13, 261 (1953). 

 "1" J. J. Fox, L. F. Cavalieri, and N. Chang, J. Am. Chem. Soc, 75, 4315 (1953). 

 ''!'' L. F. Cavalieri, J. Am. Chem. Soc, 75, 5268 (1953). 

 "2 E. L. Bennett, J. Ain. Chem. Soc. 74, 2420 (1952). 

 1" L. L. Bennett, J. Am.. Chem. Soc. 74, 2432 (1952). 



