334 ERWIN CHARGAFF 



thymus has been shown to include small amounts (0.01 to 0.1 %) of mag- 

 nesium.'*^ When cautiously prepared, these salts are obtained in the form 

 of white, tough strands of fibers, resembling asbestos, or as white fiber felts 

 after the evaporation of their frozen aqueous solution in a vacuum.'*'' In 

 addition to the cation, they contain the elements C, H, N, O, and P. The 

 determination of C and H usually does not contribute much to the charac- 

 terization of the substances, but N and P should be measured. Nitrogen is 

 best determined by the Dumas procedure, phosphorus by the Pregl-Lieb 

 method. For routine purposes, colorimetric estimation procedures for P 

 are convenient. We employ the method of King;'** compare also Jones et 

 a/.'** The sodium nucleates contain around 12 % of firmly bound water 

 which is best determined separately by drying the specimen at 60° for 3 

 hours and correcting the other analytical values for the moisture content. 

 The organic constituents should never be estimated in heated preparations. 

 (Compare also Peacocke.^*) 



A selection of analytical data is given in Table I. It will be seen that the 

 N and P values and also the~ atomic N/P ratios are in many cases in good 

 agreement with the figures calculated from the distribution of purines and 

 pyrimidines actually found in the nucleic acids of the several genera. (See 

 below. Section VII.) Owing to the several structural regularities in all 

 deoxypentose nucleic acids that will be mentioned later, the N and P 

 contents are closely similar even for preparations showing wide divergences 

 in the proportions of individual bases; they cannot be utilized for the precise 

 definition of composition, except to indicate the degree of purity. The same 

 purpose is served by the determination of the deoxypentose and pentose 

 contents which is carried out by colorimetric comparison with suitable 

 nucleic acid standards."^"* [Compare Dische, Chapter 9.] 



The physical tests usually applied are measurements of the absorption 

 spectrum in the ultraviolet and of viscosity. [See Chapters 13 and 14.] 

 While there may occur slight shifts in the position of the centers of absorp- 

 tion (between 257 and 261 m/x at pH 7), the extinction at the maximum 

 is, in my experience, almost constant. This has often been pointed out."^ • 

 116,167 When the extinction at the maximum and at pH 7 is expressed in the 

 most convenient way, namely, as the atomic extinction coefficient with 

 respect to phosphorus and designated c(P),^' preparations isolated cau- 

 tiously from a large variety of sources will show surprisingly little divergence 



'" G. Jungner, Science 113, 378 (1951). 



'*^ Lyophilized nucleic acid preparations often carry considerable static electricity, 



and this makes difficult the accurate weighing of small samples. We use a static 



eliminator containing a polonium source. 

 i«6 E. J. King, Biochem. J. 26, 292 (1932). 



'»« A. S. Jones, W. A. Lee, and A. R. Peacocke, J. Chem. Soc, 1951, 623. 

 »" E. ChargafT, Federation Proc. 10, 654 (1951). 



