OPTICAL PROPERTIES OF NUCLEIC ACIDS 



527 



10 



AlAr 



09 



08 



Mg- 



as chloride 



» Ca 



• Na*^ ) 



o Na* as sulfate 



• 8 



°m 



-4 -3 -2 



Log equivalents cations per liter 



Fig. 25. EfYect of metal cations on e(P) of calf thymus DNA. The ordinate scale 

 is the ratio of A, the absorbance in the presence of a given concentration of salt, to 

 .4o , the absorbance in the absence of salt (Shack, Jenkins, and Thompsett"). 



The nature of this role is still far from clear, but it is at least certain that 

 the "ionic" history of a DNA preparation is of great importance. 



The fact that divalent cations such as Ca++ and Mg++ stabilize DNA at much 

 lower concentrations than do monovalent ions introduces an additional practical 

 complication. Many preparations of DNA may have been dialyzed against tap water 

 containing sufficient Ca++ ions to ensure stabilization. It may be expected that such 

 preparations will be more stable at low salt concentrations than DNA that has not 

 been so treated, and that, on subsequent dialysis against distilled water, they will 

 tend to lose any remaining Na"*" ions before Ca*"'" ions. 



Certain observations which were difficult to explain are now readily accounted for 

 on the basis of the above findings. For instance it has been reported that the t(P) of 

 a preparation of DNA varies with the concentration, i.e., that Beer's law is not 

 obeyed, but that in the presence of salt «(?) is virtually constant over a wide range 

 of concentration.*^ •'*•*" 



It has also been observed that the e(P) values of a given sample of DNA in dis- 

 tilled water solution are not reproducible."" Thomas'* has shown that the extent of 



80 J. Pouyet, G. Scheibling, and H. Schwander, /. chhn. phys. 47, 417 (1950). 



