Structure of the planar complex of N4-methoxycytosine with adenine, and its relevance to the mechanism of hydroxylamine mutagenesis.

Abstract

Infrared, and 1H- and 13C-NMR spectroscopy has been applied to a study of the planar interaction in apolar media between 1-substituted N4-methoxycytosine (and the corresponding 5-methyl analogue) and 9-substituted adenines. In both chloroform and carbon tetrachloride solutions, the exocyclic N4-methoxy group of N4-methoxycytosine, and of 5-methyl N4-methoxycytosine (which is in the oxime form under these conditions), is so oriented that it is predominantly syn to the ring N(3), and neither compound forms planar auto-associates. In chloroform solution, both the N(3)-H and the C(2) = O interact weakly with the solvent, the interaction being of the nature of non-typical hydrogen bonding. The 13C-NMR chemical shift of the C(2) of N4-methoxycytosine is modified during formation of hetero-associates with 9-substituted adenine, in accordance with the C(2) = O of the former being the acceptor of an adenine amino proton. The resulting planar hetero-associate is a non-Watson-Crick type of base-pair. This was further substantiated by infrared absorption studies of the carbonyl frequency during complex formation. The results are examined in the light of the mechanism of hydroxylamine (and methoxyamine) mutagenesis.