40 D. SHUGAR 



I. Introduction 



The action of electromagnetic and particle radiations on living cells and 

 their constituent components has developed into an important branch of 

 research in almost every field of biology. With the increasing use of isotopes 

 as scientific tools, and the rapid application of nuclear energy for medical, 

 technical, and industrial purposes, it is not surprising that additional em- 

 phasis is being laid on the effects of ionizing radiations. This, however, in 

 no way detracts from the importance of the ultraviolet and visible regions 

 of the spectrum in radiobiology since the process of energy absorption is 

 specific and related to the molecular structure of the substances under in- 

 vestigation, while a good deal more is known about the physics and physical 

 chemistry of the absorption process and the dissipation of the absorbed 

 energy. It is, in fact, becoming common practice to conduct parallel studies 

 on the same systems with ultraviolet and ionizing radiations. 



The photochemistry of nucleic acids is of special interest because of their 

 important biological functions; their very high extinction in the ultraviolet 

 region of the spectrum which is most efficient in the production of muta- 

 genic, lethal, and other biological effects; the accumulated evidence from 

 action spectra that they are the immediate receptors of radiation in many 

 photobiological processes; and the recent demonstration, both by physico- 

 chemical and biological techniques, that the modifications induced in oligo- 

 and polynucleotides by ultraviolet light are at least partially reversible. 



It should be emphasized that, as in the case of ionizing radiations, the 

 photochemistry of nucleic acids is of interest not only from a biological, but 

 also from a physicochemical point of view. A good deal of work has there- 

 fore been done in which the primary aim has been to achieve appreciable 

 degradation, consequently involving the use of doses considerably in excess 

 of those required to provoke biological effects. This fact must constantly be 

 borne in mind in any attempted extrapolation of in vitro findings to the cel- 

 lular level. 



II. Principles of Photochemistry 

 1. General 



In photochemical reactions we are interested in elucidating the nature of 

 the chemical changes in a system resulting from its exposure to light. Al- 

 though the obvious fact that absorption of energy must take place for a 

 reaction to occur was formulated many years ago by Grotthus and is known 

 as the first law of photochemistry, its significance has not always been ap- 

 preciated, e.g., in relation to carbohydrates (Section V, 1) as well as in con- 

 nection with attempts to photoreactivate simple nucleic acid (NA) deriva- 

 tives (Section X). 



