OPTICAL PROPERTIES OF NUCLEIC ACIDS 



521 



f(p) 



200 



220 



240 



280 



300 



320 



260 

 Wavelength, m/i 



Fig. 21. Calculated absorption curves for a mixture of nucleosides containing 

 adenosine, guanosine, cytidine, and thymidine in the molar proportions 4:3:3:4. At 

 pH 1.5 values for adenine and guanine are employed. 



nucleotides. More recentlj' Smith and Allen^* state that the absorptivities of poly- 

 ribonucleotides containing up to 20 nucleotide residues appear to be normal, and the 

 same may be concluded of the "apurinic acid" of Chargaff and co-workers, ^^ which, 

 however, can scarcely be regarded as a normal polynucleotide. 



This absorbance anomaly therefore appears to be essentially a feature of 

 polynucleotides of relatively high molecular weight and probably denotes 

 some considerable degree of intramolecular organization. It is not due to 

 the binding of the nucleotides by glycosido-phosphate ester linkages. The 

 causes for it may therefore be of direct concern when considering theories 

 of the structure of nucleic acids. 



The work reviewed below has been mainly concerned with calf thj^mus 

 DNA, which is perhaps the most readily accessible highly polymeric nucleic 

 acid. In general, the ribonucleic acids are far less well-characterized ma- 

 terials, as a class, than the deoxyribonucleic acids. For this reason, although 

 the causes of the absorptivity anomalies may be similar in both types of 

 nucleic acid, the results of studies of the effects of pH, cations, heat, and 



«8 K. C. Smith and F. W. Allen, /. Am. Chem. Soc. 75, 2131 (1953). 



69 C. Tamm, M. E. Hodes, and E. Chargaff, J. Biol. Chem. 195, 49 (1952). 



