comparison of the electronic structure of natural nucleic bases and their analogs allows to suppose that the eas discriminates between adenine and cytosine , and uracil ( thymine ) and guanine , by electrostatic features : sign of the potential in the region of exocyclic substituents at c ( 4 ) of pyrimidines and c ( 5 ) of purines .
C0001407
adenine
C0034140
purines