Restricted rotation about the exocyclic C–N bond in nucleic acid bases

Barrier heights to rotation of amino- and methylamino-groups in several cytosine and guanine derivatives have been estimated by CNDO/2 calculations. The results are in accord with the n.m.r. studies recently reported on three cytosine derivatives. The present studies are of relevance in understanding the stabilities and interactions of methylated nucleic acids. Hindered rotation of the amino-group is also found in adenine.