METAL-STABILIZED RARE TAUTOMERS OF NUCLEOBASES .6. IMINO TAUTOMER OF ADENINE IN A MIXED-NUCLEOBASE COMPLEX OF MERCURY(II)

(1,3-Dimethyluracil-5-yl)mercury(II), (1,3-DimeU-C5)Hg-II, reacts with the model nucleobase 9-methyladenine (9-MeA) to give the mixed-nucleo base complex [Hg(1,3-DimeU-C5)(9-MeA-N6)NO3 . H2O. Hg(II) binds to the adenine via the N6 position with N1 being protonated. The neutral ade nine nucleobase is therefore present in its imino form, which represen ts the rare tautomer form of this nucleobase. The X-ray crystal struct ure analysis reveals an anti orientation of the (1,3-DimeU-C5)Hg-II en tity with respect to N1 of 9-MeA and substantial differences in nucleo base geometry as compared to the amino tautomer form of 9-MeA. These d ifferences refer to both the pyrimidine and the imidazole rings of the purine base. The possible relevance of this compound with regard to n ucleobase cross-linking in DNA and nucleobase mispairing is discussed. Attempts to demonstrate mispairing between the metalated rare tautome r and ''the wrong'' bases 1-methylcytosine (1-MeC) and 9-ethylguanine (9-EtGH) in DMSO-d(6) proved unsuccessful due to a ligand exchange pro cess at the heavy metal.