Synthesis and properties of poly(O6-methylguanylic acid) and poly(O6-ethylguanylic acid).

The nucleotide analogues, O6-methyl- and O6-ethylguanosine diphosphate, have been synthesized and polymerized to high-molecular-weight homopolymers with polynucleotide phosphorylase. The ultraviolet spectra of these polymers show marked hypochromicity, which suggests that they possess considerable secondary structures. Graphs of optical denisty vs. temperature in 0.15 M NaC1 indicate that cooperative melting occurs for both polymers, and that the secondary structure of poly(O6-methylguanosine monophosphate) is somewhat more stable than that of poly (O6-ethyl-guanosine monophosphate). Mixing experiments show that these analogue polymers no longer form helical structures with poly(C), nor do thye form helices with poly(U). We would conclude from these data that environmental mutagens and carcinogens which react at the O6 position of guanine not only disrupt normal base-pairing relationships, but may also affect the secondary structure of nucleic acids.