Abstract
Infrared spectroscopy has been employed to study possible base pair interactions, in nonpolar media, between O^6^‐methylguanine and uracil, cytosine and adenine; and between derivatives of N^4^‐hydroxycytosine and adenine. The association constants of O^6^‐methylguanine with uracil, cytosine, and adenine are well below 1__M__^–1^, whereas those for interaction of 1‐methyl‐N^4^‐methoxycytosine and its 5‐methyl derivative with adenine are identical, K = 14__M__^–1^. The significance of the latter finding is discussed in relation to the conformation of the exocyclic N^4^‐methoxy group. Quantum chemical calculations, with the aid of the perturbation method, were carried out for the interaction of O^6^‐methylguanine with uracil, cytosine, and adenine, to establish the most energetically favoured configurations for interactions of the free bases, and of the same base pairs in the B form of DNA. The role of the conformation of the exocyclic —OCH~3~ group in O^6^‐methylguanine is discussed. The relevance of both the experimental and theoretical results to mutagenesis by O^6^‐methylguanine and N^4^‐hydroxycytosine is examined.