Co(II)(Chromomycin)(2) Complex Induces a Conformational Change of CCG Repeats from i-Motif to Base-Extruded DNA Duplex

We have reported the propensity of a DNA sequence containing CCG repeats to form a stable i-motif tetraplex structure in the absence of ligands. Here we show that an i-motif DNA sequence may transition to a base-extruded duplex structure with a GGCC tetranucleotide tract when bound to the (Co(II))-mediated dimer of chromomycin A3, Co(II)(Chro)(2). Biophysical experiments reveal that CCG trinucleotide repeats provide favorable binding sites for Co(II)(Chro)(2). In addition, water hydration and divalent metal ion (Co(II)) interactions also play a crucial role in the stabilization of CCG trinucleotide repeats (TNRs). Our data furnish useful structural information for the design of novel therapeutic strategies to treat neurological diseases caused by repeat expansions.