30. PHOTOCHEMISTRY OF NUCLEIC ACIDS 59 



is suggested that the above be borne in mind in attempting to present spectra with 

 the greatest possible clarity. Summing up extinctions of the component nucleosides 

 of small oligonucleotides offers some advantages. 82 



In view of the increasing importance and use of hyperchromicity in studies on the 

 structure, photochemistry and analytical chemistry of oligo- and polynucleotides, 

 it is perhaps not out of order to specify the terms used to describe it. When the extinc- 

 tion of a given oligonucleotide is lower than that of its constituent mononucleotides, 

 it is "hypochromic" or exhibits "hypochromicity." If the oligonucleotide is hydro- 

 lyzed to mononucleotides, it exhibits "hyperchromicity." If a given oligonucleotide 

 exhibits 75% of the absorption of its constituent mononucleotides, its hypochromicity 

 is 25/100 or 25% whereas its hyperchromicity is 25/75 or 33%. 



It should be noted that hyperchromicity is by no means confined to nucleotide 

 polymers and coenzymes. The observations of Katchalski et al. 82 * on synthetic poly- 

 peptides of aromatic amino acids show the presence of spectral effects which may be 

 ascribed to hypochromicity and attaining values as high as 15%. However, probably 

 few natural proteins are hypochromic since the percentage of aromatic amino acids 

 is small; a notable exception is gramicidin where the high tryptophan content might 

 be expected to result in appreciable hypochromicity. 



5. NUCLEOPROTEINS AND VIRUSES 



Considerable new evidence, based on studies of chemical composition, 83 " 85 

 as well as X-ray diffraction data and studies of molecular models, 86 has now 

 accumulated to show that at least some of the nucleoprotamines and nu- 

 cleohistones extracted from living cells are definite complexes; and Cramp- 

 to 87 has even demonstrated how nucleohistone may be prepared by methods 

 such that dissociation cannot occur during the extraction procedure (cf. 

 Volume I, Chapter 10). Much remains to be done, however, as regards the 

 nature of the linkages between nucleic acid and protein which, in addition 

 to saltlike bonds, may apparently vary from hydrogen-bonding in a ribo- 

 nucleoprotein extracted from Escherichia coli* 8 to very firm alkali-stable 

 bonds in a deoxyribonucleoprotein from liver cell nuclei. 89 



Spectral investigations on nucleoproteins continue to be scanty and the 

 results conflicting, possibly because they have been confined to artificial 



82a E. Katchalski and M. Sela, J. Am. Chem. Soc. 75, 5284 (1953); A. Patchornik, M. 

 Sela, and E. Katchalski, ibid. 76, 299 (1954); M. Sela and E. Katchalski, ibid. 76, 

 129 (1954). 



83 P. F. Davison and J. A V. Butler, Biochim. et Biophys. Acta 21, 568 (1956); Ad- 

 vances in Enzymol. 18, 161 (1957). 



s* C. F. Crampton, W. H. Stein, and S. Moore, J. Biol. Chem. 225, 363 (1957). 



85 R. Vendrely, A. Knobloch, and W. Matsudaira, Nature 181, 343 (1958). 



86 M. Feugheiman, R. Langridge, W. E. Seeds, A. R. Stokes, H. R. Wilson, C. W. 

 Hooper, M. H. F. Wilkins, R. K. Barclay, and L. D. Hamilton, Nature 175, 834 

 (1955). 



87 C. F. Crampton, J. Biol. Chem. 227, 495 (1957). 



88 D. Elson, Biochim. et Biophys. Acta 27, 207 (1958); ibid. 36, 362 (1959). 



89 K. J. Monty and A. L. Dounce, /. Gen. Physiol. 41, 595 (1958). 



