Cytosine

Cytosine is one of the pyrimidine bases. Pyrimidine is a 6-membered aromatic ring with N-atoms in positions 1 and 3. Cytosine is derived from pyrimidine by amino substitution in the 4-position and hydroxy substitution in the 2-position. The N=C-OH tautomerizes to NH-C=O while the N=C-NH2 does not tautomerize to NH-C=NH.


Figure. The RHF/6-31G* structure of cytosine.
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