An Unprecedented Knot-like G-Quadruplex Peripheral Motif.

A knot-like G-quadruplex peripheral structure is formed by a 7-nt DNA sequence DL7 (TGTTGGT), in which six out of its seven nucleobases participate in compact base-pairing interactions. Here, the solution NMR structure of a 24-nt DNA oligonucleotide containing the DL7 sequence shows the interaction between a two-layer anti-parallel G-quadruplex core and the peripheral knot-like structure, including the construction of two sharp turns in the DNA backbone. The formation of this novel structural element highlights the intricate properties of single-stranded DNA folding in presence of G-quadruplex-forming motifs. We demonstrated the compatibility of the DL7 knot-like structure with various G-quadruplexes, which could have implications in drug design and DNA engineering.