46 D. SHUGAR 



an investigation of the fate of free radicals in yeast suspensions in connec- 

 tion with the mechanism of radiation protection. 31 A general outline of the 

 principles involved in such studies has also appeared. 32 



5. Secondary Photochemical Processes 



The excited atoms or molecules, or free radicals, produced in the primary 

 photochemical process, may subsequently undergo a wide variety of reac- 

 tions resulting in the formation of stable products. These secondary steps 

 or "dark" reactions follow the laws of chemical kinetics and in fact, studies 

 of the kinetics of the formation of photoproducts, or the disappearance of 

 reactants, form one of the important tools used for the elucidation of the 

 mechanism of a photochemical reaction. 



The various procedures used in the study of the photolytic reactions un- 

 dergone by nucleic acids and their constituents form the subject of most of 

 this chapter. Standard texts should be consulted for more general informa- 

 tion (see General Bibliography at the end of the chapter). 



III. Photochemical Techniques 



The conditions under which a photochemical experiment is carried out 

 should be reported in sufficient detail so that the results are susceptible of 

 comparison with those of other observers. The minimum of information re- 

 quired for this purpose includes the type of lamp used, whether with or 

 without filter, as well as characteristics of filter, geometrical arrangement 

 of source and irradiated substance, and concentration and layer thickness of 

 irradiated material. In view of the frequent tendency to specify only the 

 incident intensity of the source, one cannot overemphasize the fact that the 

 important quantity is the energy absorbed by the system. 



1. Sources 



The ideal source is obviously a monochromator. Very few laboratories, 

 however, dispose of such instruments suitable for photochemical work; 

 moreover, even a good monochromator will not supply the ultraviolet in- 

 tensities necessary for many photobiological experiments. In general the 

 application of such instruments has been reserved for investigations of ac- 

 tion spectra. 



For most photochemical work in the ultraviolet the use of high-pressure 

 mercury lamps is to be definitely discouraged because of the heterogeneity 

 of the emitted radiation and the lack of simple filters for isolation of specific 



31 B. Smaller and E. C. Avery, Nature 183, 539 (1959). 



32 W. Gordy, in "Information Theory in Biology" (H. P. Yockey, R. P. Platzman, 

 and H. Quastler, eds.). Pergamon Press, New York, 1958; Radiation Research, 

 Suppl. 1, 491 (1959). 



