Synthesis and characterization of a set of four dodecadeoxyribonucleoside undecaphosphates containing O6-methylguanine opposite adenine, cytosine, guanine, and thymine.

A set of four self-complementary dodecanucleoside undecaphosphates, d[CGNGAATTC(O6Me)GCG], where N = A, C, G, or T, has been synthesized by a phosphoramidite procedure. A single large-scale preparation of the nonamer d[DMT-GpApApTpTpCp(O6Me)GpCpG] was divided into four portions for synthesis of the dodecamers. The synthesis, purification (high-performance liquid chromatography), and characterization of each of these molecules are described. Each sequence forms a stable duplex, with a Tm between 19 and 26 degrees C lower than the Tm of the "parent" molecule d-(CGCGAATTCGCG). The lowest melting sequence is the N = T molecule; the overall order is N = C greater than A greater than G greater than T. Thus O6-methylation of guanine creates a region of localized instability in DNA regardless of the base opposite the lesion. This instability, which could disrupt some regulatory process or event, may be as significant as or more significant than is the mutation itself to the oncogenic process initiated by alkylating agents.