512 



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



200 



220 



240 



280 



260 

 Wavelength, m/i 



Fig. 14. Xanthosine (Johnson, unpublished). 



300 



alkaline digests of PNA. These have absorption spectra very similar to 

 those of the other isomers. In addition to the hydroxyl and amino pK's of 

 the bases and the sugar dissociation, further pX's are contributed in the 

 nucleotides by the phosphate dissociations. These would again not be ex- 

 pected to influence the spectrum greatly, but inspection of the absorbance 

 ratios in Table III for the uridylic and thymidylic acids shows that changes 

 occur between pH 2 and pH 7 which are not attributable to any other cause. 

 Separation techniques for the isomeric nucleotides have been worked out 

 only quite recently, and much of the spectroscopic work has therefore been 

 done on mixtures of unknown composition (Kerr et al.,-^ Ploeser and Lor- 

 ing,^' Chargaff and co-workers,^* Dunn and co-workers^'). Kalckar^* gives 

 data for adenylic acid-5', inosinic acid-5', and also ATP, and Kaplan et oZ.^" 

 give a curve for ITP. Data for cytidylic acids a(2') and 6(3') have been 

 published by Loring et al.*^ who found .that at pH 1 the 3'-isomer gave 

 results very close to those of cytidine. Fig. 1 shows the curves for the iso- 

 meric cytidylic acids (Cohn, private communication), and a comparison 



*^ B. Magasanik, E. Vischer, R. Doniger, D. Elson, and E. Chargaff, /. Biol. Chem. 



186, 37 (1950). 

 <' A. Deutsch, R. Zuckermann, and M. S. Dunn, Anal. Chem. 24, 1769 (1952). 

 " N. O. Kaplan, S. P. Colowick, and M. M. Ciotti, J. Biol. Chem. 194, 579 (1952). 



