LAUSR

The diversity of DNA duplex structures is limited by a binary pair of hydrogen-bonded motifs. Here we show that poly(thymine) self-associates into antiparallel, right-handed duplexes in the presence of melamine, a small molecule that presents a triplicate set of the hydrogen-bonding face of adenine. X-ray crystallography shows that in the complex two poly(thymine) strands wrap around a helical column of melamine, which hydrogen bonds to thymine residues on two of its three faces. The mechanical strength of the thymine–melamine–thymine triplet surpasses that of adenine–thymine base pairs, which enables a sensitive detection of melamine at 3 pM. The poly(thymine)–melamine duplex is orthogonal to native DNA base pairing and can undergo strand displacement without the need for overhangs. Its incorporation into two-dimensional grids and hybrid DNA–small-molecule polymers highlights the poly(thymine)–melamine duplex as an additional tool for DNA nanotechnology. Melamine is shown to associate with two poly(thymine) strands, forming antiparallel duplexes that can be used for the dynamic assembly of DNA-based nanostructures.