OPTICAL PROPERTIES OF NUCLEIC ACIDS 529 



measurements are made. The concentrations of DNA recjuired for elec- 

 trometric, conductimetric, and viscosimetric measurements may be one 

 hundred times greater than those required for spectrophotometric mea- 

 surements.^^ ■*- 



5. Effect of Other Agents on the e(P) of DNA 



Thomas^^ has shown that urea (6 M at 20° C.) is without denaturing 

 effect on DNA. Blout and Asadourian/^ working with a DNA stock solu- 

 tion in distilled water, have found that urea lowers e(P) slightly, whereas 

 plasma albumin and lysozyme have a marked effect in the same direction. 

 They also find that non-ionic compounds such as polyvinyl alcohol, sucrose, 

 and glycerol have no effect even in high concentration. It appears, however, 

 from Lawley's results^^ that ethanol increases «(?) : it is maximally effective 

 in 50% concentration while at higher or lower concentrations e(P) ap- 

 proaches its distilled water value. He has also shown that Ag+ has a specific 

 action on DNA, unlike that of other metal cations. Addition of silver nitrate 

 to a solution of DNA first lowers the e(P) value, this lowering being maxi- 

 mal for one equivalent of Ag^ per 4 atoms of DNA phosphorus. Further 

 addition causes a restoration of e(P) to the value for the DNA in distilled 

 water. The explanation for this is still obscure, but Lawley suggests that 

 the bases themselves are involved. It would be of interest to examine the 

 action of other heavy metals on the e(P) value of a nucleic acid. Such mea- 

 surements will be limited in many cases by the insolubility of the prod- 

 ucts.83.84 



6. Spectrophotometric Estimation of Nucleic Acids 



In general the spectrophotometric analysis of nucleic acids is carried out 

 by hydrolysis to free nucleotides, nucleosides, or bases, followed by their 

 separation and individual estimation, from which an estimate of the total 

 nucleic acid may be obtained. [Cf. Chapters 5-7, 9-11.] 



However, methods for direct estimation of nucleic acids by spectro- 

 photometric means have been developed. Ogur and Rosen^^ by differential 

 extraction with perchloric acid have estimated DNA and PNA in corn 

 root tips and rabbit liver in this way. Tsuboi®^ and Logan et al.^^ have used 

 trichloracetic acid in a similar method. These procedures cause considerable 

 degradation of the nucleic acids which, however, does not result in any 

 net loss of absorbing material. It is, in fact, an essential feature of these 

 methods that the nucleic acids should be degraded so that the €(P) anomaly 



"2 J. A. V. Butler and B. E. Conway, Nature 172, 153 (1953) . 



S3 E. Hammarsten, Biochem. Z. 144, 383 (1924). 



" K. G. Stern and M. A. Steinberg, Biochim. el Biophys. Acta 11, 553 (1953). 



85 J. E. Logan, W. A. Mannell, and R. J. Rossiter, Biochem. J. 61, 480 (1952). 



