Epigenetic effects of cytosine derivatives are caused by their tautomers   in Hoogsteen base pairs
  Deoxycitidine in solution exists as two tautomers one of which is an uncanonical imino one. The latter can dominate with such derivatives as 5-methyl, 5-hydroxymethyl- and 5-formylcytosine. The imino tautomer potentially is able to form a hoosteen GC base pair. To detect such pair, it is suggested to use 1H15N NMR. Formation of GC-Hoogsteen base pair with imino tautomer of cytosine can be a reason for epigenetic effects of 5-methyl- and 5-hydroxymethylcytosine. 
Denis A. Semenov
q-bio.BM