The Tautomerism of Cytosine and Hydroxycytosine. A Quantum-Mechanical Study

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Abstract
Semi-empirical studies by the MNDO and MINDO-3 methods together with ab initio studies by the Hartree Fock method, have been performed on the tautomers of cytosine (C) and hydroxycytosine (HC), in each case with full optimisation of the molecular skeleton. The results are in agreement with recent solution studies of these systems, and show that the amino form of C and the oxime form of HC are the lowest energy tautomers. Thus the property is one of the ring system rather than environment. Various conformers of the imino tautomer of 5-methyl-N(4)-hydroxycytosine were studied by MINDO/3 and PCILO calculations; although only partial geometry optimisation was performed, it seems unlikely that the molecule can be a complementary base to adenine in the Watson-Crick sense.`

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