OPTICAL PROPERTIES OF NUCLEIC ACIDS 



499 



200 



220 



240 



280 



300 



320 



260 

 Wavelength, rrtfi 



Fig. 4. Hypoxanthine (Johnson, unpublished). 



Wyatt,-^ Markham and Smith," and Loring et al.f^ and the results pre- 

 sented here agree quite well with the more recent of these. It does not seem 

 to have been observed previously, however, that both compounds have an 

 additional pK in the region of pH 0, and show marked spectral changes 

 presumably associated with the ionization of one of the ring nitrogen atoms. 

 Some methods of spectroscopic analysis have failed to take this into ac- 

 count. 



The first two references of this group also contain data for hypoxanthine 

 and xanthine, absorption curves for which have in addition been published 

 by Stimson and Reuther.^'^ 



Absorption data for cytosine, uracil, and thymine have been published 

 in recent years by Hotchkiss,^^ ^'ischer and Chargaff,^! Stimson," Wyatt,^'' 

 and Shugar and Fox.^^ Excellent data for the first two of these are given by 



" S. E. Kerr, K. Seraidarian, and M. Wargon, J. Biol. Chem. 181, 761 (1949). 



" K. K. Tsuboi and R. E. Stowell, Biochim. et Biophys. Acta 6, 192 (1950). 



24 G. R. Wyatt, Biochem. J. 48, 584 (1951). 



" R. Markham and J. D. Smith, Biochem. J. 49, 401 (1951). 



" H. S. Loring, J. L. Fairley, H. W. Bortner, and H. L. Seagran, /. Biol. Chem. 197, 



809 (1952). 

 " M. M. Stimson, /. Am. Chem. Soc. 71, 1470 (1949). 



