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



of Table II with Table III, also compiled from data kindly provided by 

 Cohn, shows that in general the nucleotide in which the phosphate is near- 

 est the base, i.e., the 2', shows the greatest differences from the correspond- 

 ing nucleoside, which might perhaps be expected. The curves for guanyhc 

 acid (Fig. 19) were determined in this laboratory. Although separation of 

 the isomers was not specifically attempted, the compound was purified by 

 ion-exchange chromatography and appears from the absorbance ratios to 

 consist very largely of the 3'-isomer. The very close resemblance to guano- 

 sine (Fig. 13) may be noted. 



The absorption curves given for 5-methylcytidylic acid (Fig. 20, from 

 Cohn^^) appear to be the only ones so far available for a deoxynucleotide. 

 These are however relative, although a very close estimate of absolute 

 values may be obtained from the corresponding nucleoside. 



III. Nucleic Acids and Polynucleotides 

 1. General 



As is to be expected from their content of purines and pyrimidines, intact 

 nucleic acids show selective absorption in the X 260-mM region. 



Spectrophotometry has been widely used for estimating the purine and 

 pyrimidine bases obtained from hydrolytic degradation of nucleic acids. 

 [Cf. Chapters 5-7, 10, and 11.] Reviewing in 1940 the small amount of prior 

 spectrophotometric work, Loofbourow" concluded that the absorptivity of 

 a nucleic acid was, to a close approximation, the sum of the contributions 

 of the constituent bases, assuming the tetranucleotide theory of their com- 

 position to be correct. The spectrophotometric analysis of an intact nucleic 

 acid would thus constitute a difficult four- or perhaps five-component analy- 

 sis. This does not prove possible, howeyer, since nucleic acids, both DNA^^* 

 and PNA, show lower absorptivities, by up to 35%, than the value cal- 

 culated from summing the molar absorptivities of the contained nucleotides 

 (as determined by separation and analysis) . Possible causes for this anomaly 

 are of interest in relation to theories of the structure of nucleic acids. The 

 reduction in total absorptivity may be regarded as arising from modifi- 

 cations of the chromophoric groups of one or more of the individual nucleo- 

 tide units in the macromolecule. Further, it appears that the absorptivity 

 value of a nucleic acid is very sensitive to its previous treatment and in 

 some respects may be more easily affected than other physical properties. 

 Although the results of investigations of these phenomena are beginning to 

 appear, it should be emphasized that complete explanation of the anomalous 



" J. R. Loofbourow, Revs. Mod. Phys. 12, 320 (1940). 



*'* The abbreviation DNA maj^ be understood to refer to the sodium salt where ap- 

 propriate. 



