Assembly of supramolecular DNA complexes containing both G-quadruplexes and i-motifs by enhancing the G-repeat-bearing capacity of i-motifs

The single-step assembly of supramolecular complexes containing both i-motifs and G-quadruplexes (G4s) is demonstrated. This can be achieved because the formation of four-stranded i-motifs appears to be little affected by certain terminal residues: a fivecytosine tetrameric i-motif can bear ten-base flanking residues. However, things become complex when different lengths of guanine-repeats are added at the 3' or 5' ends of the cytosine-repeats. Here, a series of oligomers d(XG(i)XC(5)X) and d(XC(5)XG(i)X) (X = A, T or none; i < 5) are designed to study the impact of Grepeats on the formation of tetrameric i-motifs. Our data demonstrate that tetramolecular i-motif structure can tolerate specific flanking G-repeats. Assemblies of these oligonucleotides are polymorphic, but may be controlled by solution pH and counter ion species. Importantly, we find that the sequences d(TG(i)AC(5)) can form the tetrameric i-motif in large quantities. This leads to the design of two oligonucleotides d(TG(4)AC(7)) and d(TG(Br)GG(Br)GAC(7)) that selfassemble to form quadruplex supramolecules under certain conditions. d(TG(4)AC(7)) forms supramolecules under acidic conditions in the presence of K+ that are mainly V-shaped or ring-like containing parallel G4s and antiparallel i-motifs. d(TG(Br)GG(Br)GAC(7)) forms long linear quadruplex wires under acidic conditions in the presence of Na+ that consist of both antiparallel G4s and i-motifs.