OPTICAL PROPERTIES OF NUCLEIC ACIDS 



519 



260 

 Wavelength, m/i 



Fig. 19. Guanylic acid (largely 3'-isonier) (Johnson, unpublished). 



320 



pies showed similar increases in absorption of from 5 to 15% on enzymic 

 degradation at neutral pH or on partial alkaline or acid hydrolysis. Ku- 

 nitz^- also first observed that depolymerization of DXA at pH 5 by de- 

 oxyribonuclease was accompanied by an ultimate increase in absorption at 

 2G0 m/x of nearly 30%, and increases of a similar order were reported for 

 both DNA and PXA by Tsuboi.^^ 



The relation between the absorptivity of the intact nucleic acids and the 

 value obtained by summing the absorptivities of the constituent nucleotides 

 was first pointed out, for a number of PXA samples, by Magasanik and 

 Chargaff." They found that the absorbance of the total alkaline hydrolysis 

 products was invariably greater by some 24-37 % than that of the original 

 sample, and that if, after partial enzymic hydrolysis of the PXA, the prod- 

 ucts were separated into dialyzable and nondialyzable fractions, the in- 

 crease in absorbance or "hyperchromic effect" on total alkaline hydrolysis 

 was almost negligible for the former and very marked for the latter. These 

 latter polynucleotide "cores" were especially rich in guanylic acid, and it 

 appeared that this hyperchromic effect might be especially associated with 



«2M. Kunitz, J. Gen. Physiol. 33, 349 (1950). 



63 K. K. Tsuboi, Biochim. et Bwphrjs. Acta 6, 202 (1950). 



64 B. Magasanik and E. Chargaff, Biochim. et Biophxjs. Acta 7, 396 (1951). 



