60 D. SHUGAR 



complexes; e.g., Seibert 90 finds that such complexes exhibit decreases in 

 extinction from additivity, while Nurnberger 91 finds only increases. Kla- 

 merth 92 has studied the increase in absorption of DNA-histone complexes 

 at 259 m^u with increasing histone concentrations and showed the existence 

 of two points of inflection, the first at an N/P ratio of 3.8 characteristic of 

 normal DNA histones 83 and a second at an N/P ratio of 9, which is the limit 

 of histone binding of the complex. It must, therefore, be emphasized that 

 spectral investigations of artificial nucleoproteins should be supplemented 

 by additional independent evidence of complex formation, such as by light 

 scattering, sedimentation, diffusion, and electrophoresis. 



In general it appears that, in the region 240-300 mp. which is most used 

 for photochemical investigations, the deviations from additivity in a nu- 

 cleoprotein complex are not important enough to cause apprehension. At 

 shorter wavelengths peptide bond absorption begins to predominate, a fact 

 of considerable significance in studies on action spectra and on the mech- 

 anism of ultraviolet inactivation of viruses (Section IX). 



The nature of the linkage between nucleic acid and protein can pro- 

 foundly influence the photochemical behavior of the former, the most strik- 

 ing example being that of the tobacco mosaic virus (TMV), where it has 

 been shown that this is the source of differences in sensitivity of different 

 strains and of previous failures to observe photoreactivation (Section X). 



In the case of viruses, quantitative photochemical studies must also sur- 

 mount the hurdle of scattered light which may be appreciable because of 

 the high molecular weights of these particles. The usual procedure has been 

 to measure spectrophotometrically the scattered light above 320 nux, where 

 the virus does not specifically absorb, and applying the Rayleigh law of 

 scattering, to calculate graphically the amount of scattering at shorter 

 wavelengths. 93 ' 94 For tobacco mosaic virus the amount of scattering at 260 

 m/i has been calculated in this way to be about 16 %. 93, 95 A more direct 

 approach has involved the comparison of the optical densities of solutions 

 of x-protein from infected tobacco plants, one at pH 7.3 at which the molec- 

 ular weight of the protein is low, and the other at pH 5.3 where it is polymer- 

 ized into rods of the approximate size of TMV; the amount of scattered 

 light calculated in this way is over 20 %. 96 Methods based on calculation of 



90 F. B. Seibert, Discussions Faraday Soc. 13, 251 (1953). 



91 J. I. Nurnberger, in "Analytical Cytology" (R. C. Mellors, ed.), Chapter 4. 

 McGraw-Hill, New York, 1955. 



92 O. Klamerth, Z. Nalurforsch. 12b, 186 (1957). 



93 G. Schramm and H. Dannenberg, Ber. 77, 53 (1944). 



94 It. Dulbecco, J. Bacteriol. 59, 329 (1950). 



95 K. K. Reddi, Biochim. el Biophys. Acta 24, 238 (1957). 



96 A. D. McLaren and W. N. Takahashi, Biochim. et Biophys. Acta 32, 555 (1959). 



