232 



NUCLEIC ACIDS AIS'D RELATED COMPOUNDS 



H 



/6^ 



Ni 



CH 



H 



Ni 



HC2 ^CH 

 N 



pyrimidine 



sC 



HC 



4C 





8CH 



purine 



Neither of them occurs free in nature, although a nucleoside of purine (1) occurs in 



Clitocybe nebular is. 



The two purines and the pyrimidine found in all nucleic acids are as follows: 



N^ ^N 



adenine guanine cytosine 



Thymine and uracil found respectively in deoxyribonucleic acid and ribonucleic acid are: 



thymine 



uracil 



Although structures are shown with fixed double bonds, it should be recognized that in 

 many cases their reactivities can be understood better if they are regarded as resonance 

 hybrids. Tautomerization also may play an important role. Thus uracil may be repre- 

 sented by three different contributing structures: 



IH OH 



HN Y ^ \^ ¥ < > II 1^ 



H 



TAUTOMERISM 



RESONANCE 



