OPTICAL PROPERTIES OF NUCLEIC ACIDS 



511 



16 



14 



12 



>; 10 



200 



220 



240 



260 



Wavelength, m/i 



280 



300 



320 



Fig. 13. Guanosine (Johnson, unpublished). 



out b\- comparing their absorption spectra with those of the corresponding methyl- 

 substituted compounds (Gulland et oZ." •'"■■'*). 



It may perhaps be pointed out here that the deduction of Press and Butler/* 

 on spectroscopic grounds, that the ring system of guanosine remains intact after 

 treatment with di(2-chloroethyl)methylamine seems ill-founded. From the data 

 given, there seems no reason to assume that the iminazole ring is unbroken, since 

 pyrimidine derivatives frequently possess absorption maxima at longer wavelengths 

 than purines. 



3. Nucleotides 



The separation of the phosphate group in a nucleotide from the effective 

 ultraviolet chromophore by the saturated carbon chain of the sugar residue 

 may be expected to ensure that its effect on the absorption characteristics 

 will be small. This is in fact the case, and many nucleotides may be regarded 

 as barely distinguishable from their parent nucleosides in the present state 

 of ultraviolet spectroscopic technique. 



The phosphate group may be substituted in the 2'-, 3'-, or 5'-position in 

 ribonucleotides, and Markham and Smith" have recently shown that cyclic 

 2',3'-monohydrogen phosphate esters may also be isolated from enzymic or 



^s J. M. Gulland and E. R. Holiday, /. Chem. Soc. 1936, 765. 

 " E. M. Press and J. A. V. Butler, /. Chem. Soc. 1952, 626. 

 " R. Markham and J. D. Smith, Biochem. J. 52, 552 (1952). 



