54 D. SHUGAR 



decomposition of dihydropyrimidines to /3-ureido acid derivatives. 60 Accord- 

 ing to Batt et al. 60 the spectral changes for dihydrouracil (and the dihydro 

 derivatives of thymine and orotic acid) are consistent with an enol form 

 at alkaline pH. Studies in progress in our laboratory (Janion and Shugar) 

 indicate that the transition from the neutral to the alkaline curve is due 

 to ionization of one of the carbonyl groups. Dihydrodimethyluracil exhibits 

 a similar maximum at 225 nnx in neutral medium, but this is not modified 

 by alkalization prior to decomposition (cf. dimethylbarbituric acid 61 ). 



The preparation of dihydrocytosine derivatives is more difficult because 

 of the danger of overdehydrogenation and deamination, even at neutral 

 pH. 64 Figure 6 exhibits the absorption spectra at neutral pH of deoxycyt- 

 idine and a sample of impure dihydrodeoxycytidine, so that extinction 

 coefficients for the latter should not be considered as accurate. The absorp- 

 tion spectrum of dihydrocytosine is similar in shape to that of dihydrode- 

 oxycytidine, but the maximum is at 237.5 nux. Since position 3 in dihydrode- 

 oxycytidine is blocked, it is clear that the chromophore principally 

 responsible for the peak at about 240 m/x is 



I I 



0=C— N=C— 



It is of interest in this connection to recall that cytosine nucleosides (but 

 not cytosine or 3-methylcytosine) show a point of inflection 65 ' 66 which, in 

 the case of pyranosylcytosines, 66 cytidine-2',3'-phosphate, 39 ' 67 and iso- 

 propylidenecytidine (Wierzchowski and Shugar, unpublished), is resolved 

 into a clearly defined maximum at about 236 nnx, due most likely to the 

 above chromophore and resulting from some type of interaction between 

 the pyrimidine and carbohydrate rings (see also Section V, 4, b). 



4. Hyperchromicity in Oligo- and Polynucleotides 



The fractional increase in absorption (hyperchromicity) accompanying 

 the degradation of nucleic acids has already been discussed (Volume I, 

 Chapter 14), particularly with reference to its relationship to nucleic acid 

 structure. Some of the conclusions reached at that time with respect to 

 hyperchromicity in small oligonucleotides require modification in the light 

 of additional evidence which has since become available. Furthermore the 

 development of chemical and enzymic procedures for the synthesis of homo- 

 and hetero-oligonucleotides (see Chapter 31) now make it possible to in- 

 vestigate with more certainty, and considerable simplification, the factors 

 contributing to hyperchromicity. This, in turn, has already proved useful 



65 D. Shugar and J. J. Fox, Biochim. et Biophys. Acta 9, 199 (1952). 



66 J. J. Fox and D. Shugar, Biochim. et Biophys. Acta 9, 369 (1952). 



67 D. Shugar and K. L. Wierzchowski, Bull. acad. polon. sci., Classe II, 6, 283 (1958). 



