30. PHOTOCHEMISTRY OF NUCLEIC ACIDS 45 



or electron spin resonance (ESR) absorption, as it is now referred to, which 

 involves measurements of the absorption of energy in the microwave region 

 of the spectrum as a result of the interaction between an applied variable 

 magnetic field and the magnetic moments of the unpaired electrons in the 

 sample under investigation. 1722 



Subsequent improvements in technique have made possible the applica- 

 tion of this method to wet samples, as a result of which the existence of free 

 radicals has been demonstrated in isolated enzyme systems, viz., various 

 dehydrogenase and xanthine oxidase systems, as well as in a number of ac- 

 tively metabolizing cells including isolated chloroplasts, chlorella cells, bac- 

 teria, and mammalian tissues. 23 " 26 Ehrenberg and Ludwig 25 have, however, 

 emphasized the importance of obtaining adequate kinetic data before con- 

 cluding that the free radicals observed are real intermediates in the systems 

 studied. Further refinements in technique are obviously desirable in order 

 to make possible the precise identification of the free radicals in living cells 

 and isolated enzyme systems. 



It is therefore clear from the above that the study of free radicals far 

 transcends the fields of photochemistry and radiation chemistry and is 

 bound to assume a role of increasing importance in studies on the metabolic 

 activities of living cells. This in turn enhances the value of photochemical 

 studies of biologically important molecules, since optical excitation is prob- 

 ably the most useful and precise method for attaining known excited elec- 

 tronic states. 



Finally, mention should be made of the fact that electron spin resonance 

 has been used to study radiation induced damage in proteins 27 ■ 28 and ir- 

 radiated nucleic acids and their derivatives. 29 ' 30 It has also been applied to 



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18 E. Zavoisky, J. Phys. (U. S. S. R.) 9, 211 (1945). 



19 N. N. Semenov, "Some Problems in Chemical Kinetics and Reactivity." Acad. 

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 1958. 



20 J. E. Wertz, Chem. Revs. 55, 829 (1955). 



21 P. B. Sogo and B. M. Tolbert, Advances in Biol, and Med. Phys. 5, 1 (1957). 



22 D. J. E. Ingram, "Free Radicals as Studied by Electron Spin Resonance." But- 

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23 B. Commoner, J. J. Heise, and J. Townsend, Proc. Natl. Acad. Sci. U. S. 42, 710 

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24 B. Commoner, J. J. Heise, B. B. Lippincott, R. E. Norberg, J. V. Passoneau 

 and J. Townsend, Science 126, 57 (1958). 



25 A. Ehrenberg and G. D. Ludwig, Science 127, 1177 (1958). 



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27 W. Gordy and H. Shields, Radiation Research 9, 611 (1958). 



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30 J. W. Boag and A. Muller, Nature 183, 831 (1959). 



