Novel phthalimide nucleosides for the specific recognition of a CG Watson–Crick base pair

In an effort to extend the triple helix recognition code we have synthesized various substituted phthalimide-derived nucleosides that can recognize a CG Watson–Crick base pair. NMR experiments performed on the free nucleosides in methylene chloride at lowered temperatures indicate the strength and extent of H-bonding to the cytosine amino group of a CG base pair which strongly depend on the substituent of the phthalimide nucleobase. Consistent with the formation of an additional hydrogen bond to N7 of guanine, ureido-substituted nucleoside analogs show a higher overall affinity as compared to the 3-aminophthalimide nucleoside.