Synthesis and properties of H-ras DNA sequences containing O6-substituted 2'-deoxyguanosine residues at the first, second, or both positions of codon 12.

Nine 16-base oligodeoxyribonucleotides having the sequence of codons 9 through the first base of codon 14 of the rodent H-ras gene, i.e., 5'-d(GTGGGCGCTG*G*AGGCG)-3', have been synthesized containing either an O6-methyl- (G* = m6G), O6-ethyl- (G* = e6G), or the newly described O6-benzyl-2'-deoxyguanosine residue (G* = b6G) at position 10 and/or 11 from the 5'-end. The conversion of the protected O6-substituted 2'-deoxyguanosine derivatives to the corresponding 3'-[O-(2-cyanoethyl) diisopropylphosphoramidites] and their incorporation into oligodeoxyribonucleotides were conveniently accomplished by using an "in situ" activation approach and automated phosphite triester synthetic methods. These oligomers were characterized by enzymatic digestion to their component nucleosides and were shown to be free of detectable contamination by known nucleoside impurities that can be produced during these syntheses. The melting behavior and circular dichroism spectra are described for duplexes of the nine O6-substituted 2'-deoxyguanosine containing oligomers paired with the complementary strand 5'-d(CGCCTCCAGCGCCCAC)-3', and these data have been compared with those for the "wild-type" unsubstituted duplex.