Binding of Naphthyridine Carbamate Dimer to the (CGG)_n Repeat Results in the Disruption of the G-C Base Pairing

摘要

The expansion of the (CGG)_n trinucleotide repeat in the FMR1 gene causes the neurological disorder fragile X syndrome.The molecular basis for the (CGG)_n expansion involves the formation of a metastable hairpin structure consisting of continued 5'-CGG-375'-CGG-3' triads,in which a G-G mismatch is flanked by two G-C base pairs.We recently reported the remarkable binding of naphthyridine azaquinolone (NA) to the 5'-CAG-375'-CAG-3' triad in the hairpin form of the (CAG)_n repeat,where the A-A mismatch was flanked by G-C base pairs.The ligand-bound structure was intriguing because 1) two NA molecules were bound to a single CAG/CAG triad which 2) induced the cytosine-which was hydrogen-bonded to the guanine-to flip out from the base stack (Figure 1).The NA-immobilized sensor was useful for the rapid diagnosis of the (CAG)_n repeat length.We have reported a series of ligands binding to a G-G mismatch,and therefore the remarkable structure of NA bound to the CAG/CAG triad prompted us to investigate the mode of ligand binding to the CGG/CGG triad and,hence,the possibility of the cytosine flipping out in the ligand-bound complex.Herein,we report that naphthyridine carbamate dimer (NC) binds to a single CGG/CGG triad with exclusively 2:1 NC/triad stoichiometry.The binding of NC to the CGG/CGG triad induced the disruption of the guanine-cytosine base pairing in the triad,and made the cytosine susceptible to the subsequent chemical cleavage reaction initiated by addition of hydroxylamine.