OPTICAL PROPERTIES OF NUCLEIC ACIDS 545 



noteworthy that unhke other biological macromolecules with acidic func- 

 tional groups, PNA is a unique substrate for some basic dyes in that it re- 

 presses dimerization and hence the change in dye color known as metachro- 

 masy. The subject has been discussed by Michaelis."^ [Cf. Swijt, Chapter 

 17.] 



V. Infrared Absorption Spectra 



The application of infrared absorption techniques to the study of nucleic 

 acids is relatively new as in Loofbourow's extensive review article published 

 in 1940," on the investigation of materials of biological interest by physical 

 methods, there is not a single reference to this topic. Even Schlenk's account 

 of nucleic acid chemistry which appeared in 1949^^^ cites only one paper, 

 that of Blout and Fields^*' published in 1948. Subsequent investigations on 

 nucleic acids proper have been reviewed very recently by Fraser.^^" Several 

 extensive studies, however, of simple pyrimidines and related compounds 

 have been made and are briefly noted here. 



Broadly speaking, infrared absorption methods have been used for two 

 different purposes in the nucleic acid field. On the one hand, the wealth of 

 detail in an infrared spectrum may be used as a "finger-print" to identify a 

 particular compound, and to detect it in mixtures. The infrared absorption 

 spectrum may also be used as a proof of identity between synthetic and 

 natural specimens of the same compound, especially when the existence of 

 isomers with very similar properties results in other characterization meth- 

 ods being inadequate for such a purpose. Many examples of this application 

 can be found in the recent literature dealing with the synthesis of nucleo- 

 sides and nucleotides. For example, Brown and Todd^" were able to dis- 

 tinguish between natural and synthetic adenylic acids a and h and muscle 

 adenylic acid (adenosine-5'-phosphate) by their infrared absorption spectra, 

 obtained on mulled samples, and the isomeric cytidylic acids can also be 

 identified in spite of the additional complication of polymorphism.^^' A 

 strong characteristic band free from overlapping can be used for the esti- 

 mation of a compound in mixtures if suitable solvents are available, though 

 even mull spectra are of analytical value. ^^^ Work of this sort entails the use 



"' L. Michaelis, Nucleic acids and nucleoproteins, Cold Spring Harbor Symposia 



Quant. Biol. 12, 131-142 (1947); see also G. Oster and H. Grimsson, ref. 60. 

 1" F. Schlenk, Chemistry and enzymology of nucleic acids, Advances in Emymol. 9, 



455-535 (1949). 

 1" E. R. Blout and M. Fields, Science 107, 252 (1948). 

 IS" R. D. B. Fraser, The infra-red spectra of biologically important molecules, Progr. 



Biophys. and Biophys. Chem. 3, 47-60 (1953). 

 1" D. M. Brown and A. R. Todd, J. Chem. Soc. 1951, 44. 

 162 R. J. C. Harris, S. F. D. Orr, E. M. F. Roe, and J. F. Thomas, J. Chem. Soc. 1953, 



489. 



