Solid-phase synthesis of the self-complementary hexamer d(c7GpCpc7GpCpc7GpC) via the O-3'-phosphoramidite of 7-deaza-2'-deoxyguanosine.

The synthesis of the O-3'-phosphoramidite of a suitably protected 7-deaza-2'-deoxyguanosine (c7G) which is an isostere of 2'-deoxyguanosine is described. The phosphoramidite of the modified nucleoside was used in the synthesis of the self-complementary hexamer d(c7GpCpc7GpCpc7GpC) on functionalized silica gel in a mini-reactor. As expected from the parent hexamer d(GpCpGpCpGpC) the isosteric d(c7GpCpc7GpCpc7GpC) exhibits a rigid secondary structure (22% hypochromicity at 280 nm) and forms a duplex in 1 M aqueous sodium chloride solution. Due to the altered pi-electron system of the pyrrolo[2,3-d]pyrimidine nucleobase, which affects base stacking and hydrogen bonding, the Tm of the modified duplex is decreased by 10 degrees C compared to that of the parent purine hexamer. Moreover, it is expected that the incorporation of c7G influences the pitch of the helix.