528 



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



1.30 



1.20 



110 



1.00 



"Undenatured" preparation in lO""^ M NaCI 

 Preparation partially denatured by dilution to 10""'' M 



NaCI before titration 

 Preparation partially denatured by dilution as above. 



brought back to IQ-^M NaCI before titration 



PH 



Fig. 26. The variation of e(P) with pH of "undenatured" and partially denatured 

 preparations of calf thymus DNA (Thomas''*). 



The ordinate scale is the ratio, Ra , of the observed e(P) value to the minimum 

 value (measured at pH 6 in 10"^ M NaCI solution) . The titration in each case is carried 

 out from the higher pH value. 



denaturation in such solutions depends on the time lapse between the preparation of 

 the solution and examination. 



A further protective effect of salts against heat denaturation of DNA solutions 

 has been observed by Thomas.'* -^^ For calf thjrmus DNA in sodium chloride solutions 

 of various concentrations, some denaturation occurs at room temperature at <10~^ M 

 and below 70° in 10"^ M. Denaturation is strongly inhibited even at 100° C. in 10"^ M 

 and M solutions. It is claimed that heat denaturation occurs stepwise and that the 

 critical temperature for each step varies for DNA from different sources. Thus DNA 

 from starfish testis in 10"^ M sodium chloride undergoes the first step in denaturation 

 at 55° C. compared with 70° C. for that from calf thymus. 



The bearing of these results on theories of the structure of DNA may 

 become apparent when they are correlated with the study of other physical 

 properties. This may not be a simple matter because of the very different 

 nucleic acid concentrations at which spectrophotometric and other physical 



81 R. Thomas, Trans. Faraday Soc. 50, 290 (1954). 



