2‑Amino-1,8-naphthyridine Dimer (ANP77), a High-Affinity Binder to the Internal Loops of C/CC and T/CC Sites in Double-Stranded DNA

Small molecules targeting DNA regions with structural fluctuation are an important class of molecule as chemical probes for studying the role of these structures in biological systems and the development of neurological disorders. The molecule ANP77 we described here, where a three-atom linker connects two 2-amino-1,8-naphthyridines at the C7 position, was found to form stacked structure with protonation of naphthyridine at low pH, and bound to the internal loop consisting of C/CC and T/CC in double-stranded DNA with affinities of 4.8 and 34.4 nM, respectively. Mass spectrometry and isothermal titration calorimetry analyses determined the stoichiometry for the binding as 1:1, and chemical footprinting with permanganate and NMR structural analysis revealed that the T in the T/CC was forced to flip out toward an extrahelical position upon ANP77 binding. Protonated stacked ANP77 interacted with two adjacent cytosines through hydrogen bonding and occupied the position in the duplex by flipping out the C or T opposite CC. Finally, this study demonstrated the potential of ANP77 for binding to the sequences of biological significance with the TG­(T/C)­CC repeat of the PIG3 promoter and the telomere repeat CCCTAA.