530 G. H. BEAVEN, E. R. HOLIDAY, AND E. A. JOHNSON 



is completely abolished, thus enabling a direct correlation between ab- 

 sorptivity and concentration to be made. It is evident from Ogur and 

 Rosen's analyses that the c(P) of yeast PNA from different sources, after 

 such degradation, is remarkably reproducible, having a value of 10.800 

 (in A^" perchloric acid) with a standard deviation of 1.3%. Their correspond- 

 ing figure for thymus DNA is 8,780, with a standard deviation of 2.3%. 

 Such values of c(P) will vary with the purine and pyrimidine contents of 

 the particular nucleic acid, so that in order to apply the method generally 

 this e(P) value must be established in each case. [Cf. Chapter 10; Chapter 

 11.] 



7. NUCLEOPROTEINS 



The ultraviolet absorption spectrum of a nucleoprotein is composed of 

 contributions from both constituents and therefore differs from that of the 

 nucleic acid alone. The protein absorption,^^ which depends mainly on its 

 content of aromatic amino acid residues, is maximal at ca. X 280 m/x, and 

 rises again at shorter wavelengths from a minimum at ca. X 250 mju. On a 

 weight basis the peak absorption of protein will be only about one-tenth or 

 less that of nucleic acid, so that for a nucleoprotein containing, say, 40% 

 of nucleic acid, the absorption contribution of the latter therefore dominates 

 the collective absorption curve, and the contribution of the protein usually 

 appears as an inflection at ca. X 280 m^ while the nucleic acid minimum at 

 ca. X 230 m^ is shifted to longer wavelength. Since the nucleic acid content 

 of nucleoproteins varies over a wide range (e.g., 5-40% for some plant 

 viruses,^^ while the absorption of some of the proteins associated with 

 nucleic acids tends to be very low, the spectra of nucleoproteins will ob- 

 viously vary between rather wide limits; many examples can be found in 

 the literature.^^ '^^ 



Various suggestions have been made regarding the possible mode of combination 

 of the nucleic acid and protein moieties of nucleoprotein. Astbury'" pointed out that 

 the ca. 3.4-A. X-ray diffraction spacing of nucleic acids was comparable with the 

 side-chain spacing of a fully extended /3-type polypeptide chain and suggested that 

 this correspondence might allow salt-like electrostatic bonds to be formed between 

 the phosphate groups of the Astbury polynucleotide chain model and the guanidino 

 groups of arginine side chains, which form a large proportion of the amino acid 

 residues in basic proteins of the histone and protamine types. Support for this view 



86 G. H. Beaven and E. R. Holiday, Advances in Protein Chem. 7, 319-386 (1952). 

 8^ J. P. Greenstein, Advances in Protein Chem. 1, 209 (1944). 



88 R. Markham, R. E. F Matthews, and K. M. Smith, Nature 162, 88 (1948). 



89 T. Caspersson, "Cell Growth and Cell Function." Norton, New York, 1951; 

 Symposia Sac. Exptl. Biol. 1, 127-151 (1951). 



9» W. T. Astbury and F. O. Bell, Nature 141, 747 (1938); see also W. T. Astbury, 

 Symposia Soc. Exptl. Biol. 1, 66-76 (1951). 



