522 



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



10.000 



8,000 



m 



6,000 



4,000 



2,000 



200 



220 



240 260 280 

 Wavelength, m^ 



300 



Fig. 22. Calf thymus DNA (Shack and Thompsett^") . 



320 



dehydration on the €(P) of PNA will be difficult to interpret until an un- 

 denatured form (Cf. Kay and Dounce^^) can be unequivocally recognized. 



3. Effect of pH on the €(P) of DNA 



In Fig. 21 the absorption curve of a mixture of nucleotides corresponding 

 in nitrogen base content to a typical DNA is shown at three pH values. 

 In Fig. 22 is shown the absorption curve of a calf thymus DNA preparation 

 under similar conditions. These curves clearly indicate that the marked 

 changes in absorptivity of DNA with change of pH cannot be attributed 

 simply to ionization of the purine and pyrimidine bases. The effects of 

 change in pH when summed for the four principal nucleotides in a nucleic 

 acid (Fig. 21) result in a small change in absorptivity and a slight shift in 

 wavelength, whereas there are gross changes for DNA itself under similar 

 conditions. Further it can be seen that on exposing DNA to acid or alkali 

 its absorption curve approaches more closely to the calculated one. It is 

 noteworthy, however, to find that at least part of the well-known anomalous 

 electrometric titration curve of DNA"" may also be obtained spectro- 



" J. Shack and J. M. Thompsett, J. Biol. Chem. 197, 17 (1952). 



'1 E. R. M. Kay and A. L. Bounce, J. Am. Chem. Soc. 75, 4041 (1953). 



" J. M. Gulland, D. O. Jordan, and H. F. W. Taylor, /. Chem. Soc. 1947, 1131. 



" W. A. Lee and A. R. Peacocke, /. Chem. Soc. 1951, 3361. 



