OPTICAL PROPERTIES OF NUCLEIC ACIDS 549 



(4) Ionic group frequencies: 



PO4 ca. 1080 and 980 cm -1 



The large number of different atomic groupings in a biological macro- 

 molecule such as nucleic acid gives rise to a very complex set of characteris- 

 tic and skeletal frequencies so that only a few of the more important can be 

 assigned with any certainty. The situation is complicated by overlapping of 

 bands and by severe broadening of those bands arising from groups which 

 may be hydrogen-bonded. The most detailed analyses have been made on 

 the spectra of proteins and related compounds/^^ and the interpretation of 

 nucleic acid spectra has followed along essentially similar lines, extended to 

 include the structural features peculiar to these compounds. 



Blout and Fields"^ ■^" find that the sodium salts of PNA and DNA can be 

 distinguished by their infrared absorption spectra, especially at frequencies 

 lower than ca. llOOcm."^ Fraserand Chayenhave used microspectrographic 

 methods^®' to extend this finding to intact tissue sections. The specific 

 frequencies recognized by these workers are 860, 916, 969, and 997 cm.~' 

 for PNA, and 895, 930, and 967 cm.~' for DNA, as confirmed by selective 

 extraction procedures. Since the technique can be made at least semiquan- 

 titative, it offers the possibility that the two types of nucleic acid can be 

 differentiated and estimated separately in situ in biological material. A 

 further advantage is that any mononucleotide absorption contributions 

 can be distinguished and evaluated separately. Quantitative infrared micro- 

 spectrographic procedures may therefore prove to be a valuable supplement 

 to the more refined ultraviolet methods which have been given so much 

 more attention in the past. 



Macroscopic tissue sections have been studied by Blout and Mellors,^^^ the sam- 

 ples being evenly wetted with high-molecular-weight fluorocarbon or hydrocarbon 

 (mineral) oil to reduce scattering. These authors were particularly interested in the 

 possibility of clinical diagnostic applications, and observed that a band of 9.3 m 

 (1075 cm."'), which they associated with nucleic acid, was more prominent in can- 

 cerous than in healthy mammary tissue. They also found definite differences in the 

 spectra of fixed and unfixed samples of the same tissue. 



Cavalieri, Kerr, and Angelos,'^^ in a study of the enzyme-resistant residues of 

 various PNA samples, concluded that the infrared absorption spectra of mulls were 

 not sufficiently characteristic to reveal differences in macromolecular structure, al- 

 though intact nucleic acids could be differentiated from mixtures of mononucleotides. 

 More recently, however, Rowen'" has examined the effect of treatment with deoxy- 

 ribonuclease on the infrared absorption spectra of cast films of DNA (Fig. 28). The 



'" G. B. B. M. Sutherland, Infrared analysis of the structure of amino acids, poly- 

 peptides and proteins. Advances in Protein Chem. 7, 291-318 (1952). 

 '«5 R. D. B. Fraser and J. Chayen, Exptl. Cell Research 3, 492 (1952). 

 i«6 E. R. Blout and R. C. Mellors, Science 110, 137 (1949). 



