Topology specific stabilization of promoter over telomeric G-quadruplex DNAs by bisbenzimidazole carboxamide derivatives.

Various potential G-quadruplex forming sequences present in the genome offer a platform to modulate their function by means of stabilizing molecules. Though G-quadruplex structures exhibit diverse structural topologies, the presence of G-quartets as a common structural element makes the design of topology specific ligands a daunting task. To address this, the subtle structural variations of loops and grooves present in the quadruplex structures can be exploited. To this end, we report the design and synthesis of quadruplex stabilizing agents based on bisbenzimidazole carboxamide derivatives of pyridine, 1,8-naphthyridine, and 1,10-phenanthroline. The designed ligands specifically bind to and stabilize promoter quadruplexes having parallel topology over any of the human telomeric quadruplex topologies (parallel, hybrid, or antiparallel) and duplex DNAs. CD melting studies indicate that ligands could impart higher stabilization to c-MYC and c-KIT promoter quadruplexes (up to 21 °C increment in Tm) than telomeric and duplex DNAs (ΔTm ≤ 2.5 °C). Consistent with a CD melting study, ligands bind strongly (Kb = ∼10(4) to 10(5) M(-1)) to c-MYC quadruplex DNA. Molecular modeling and dynamics studies provide insight into how the specificity is achieved and underscore the importance of flexible N-alkyl side chains attached to the benzimidazole-scaffold in recognizing propeller loops of promoter quadruplexes. Overall, the results reported here demonstrate that the benzimidazole scaffold represents a potent and powerful side chain, which could judiciously be assembled with a suitable central core to achieve specific binding to a particular quadruplex topology.