184 RADIATION HIOLOGY 



by absorption. For simple spherical particles of diameter <X/1() such 

 scattering may be expected to vary as l/\*. For larger and for more 

 irregularly shaped particles, the variation of scattering power with angle 

 and with wave length depend intimately on the particle size and shape; 

 this is, in fact, the basis of molecular size and shape determination by 

 means of light scattering (Oster, 11)48; Doty and Steiner, 1950). 



In any event, scatt(M'ing will tend to obscure seriously the true absorp- 

 tion spectrum (Schramm and Dannenburg, 1944). If it is necessary to 

 determine the absorption of turbid media or coarse structures, the effects 

 of scattering may be minimized by use of a detector designed to capture 

 as much scattered light as possible (Caspersson, 1950) and by use of a fluid 

 medium with a refractive index as closely matched to that of the scatter- 

 ing substance as possible (Mitchell, 1950). In some instances it is pos- 

 sible to introduce a reasonable correction factor by extrapolation of data 

 from wave length outside the absorption band (Trciber and Schauen- 



stein, 1949). 



LOCAL CONCENTRATIONS 



In general, solutions will have a uniform distribution of the absorbing 

 substance, but this situation is not necessarily true of cellular structures. 

 If the substance tends to molecular association and deviation from Beer's 

 law, the presence of local concentrations may appreciably alter the 

 absorption spectrum. In addition to this potential effect, the aggregation 

 of the absorbing molecules into discrete groups (possibly submicroscopic) 

 will influence the absorption by virtue of the nonabsorbing "holes" left 

 between the absorbing groups. The effect on the spectrum in this 

 instance is similar to that described in the contingency of marked dichro- 

 ism. If there is an appreciable chance of a photon passing through the 

 specimen without encountering one of the postulated absorbing centers, 

 then there will be a maximum possible absorption, independent of the 

 amount of absorbing substance present. 



ULTRAVIOLET ABSORPTION SPECTRA OF IMPORTANT BIOLOGICAL 



SUBSTANCES 



Since the principal role of absorption spectra in radiation biology is to 

 serve, within the limits suggested in the preceding section, as a key to the 

 interpretation of the action spectra for various photobiological effects, it 

 is useful to have a summary of the absorption characteristics of the 

 principal known ultraviolet chromophorcs in living systems (Erode, 

 1946; Morton, 1942; Loofbourow, 1940, 1943; Kllinger, 1937, 1938; 

 Miller, 1939). 



PROTEINS AND AMINO ACIDS 



It is convenient to consider separately the ultraviolet absorption of 

 proteins in the region below 2400 A and in the region abo\'e — generally 



