Mechanical Properties of DNA Double-Crossover Motif

The double-crossover (DX) motif is a key building block of DNA nanostructures. It connects two double helices by two closely spaced Holliday junctions. Despite its prominent importance, the structure and elasticity of the DX motif is not fully understood. Here we employ extensive all-atom molecular dynamics (MD) simulations of an antiparallel DNA DX motif with two full turns between crossovers to infer its global structure and deformability. We quantitatively reproduce the experimentally known two-fold increase of bending stiffness upon incorporation of a DNA duplex into the DX motif, and find out that its stretching and twisting stiffness are only slightly influenced. To further describe the motif, we define four effective rigid bodies, each involving several base pairs flanking the Holliday junctions from the outside, and consider internal coordinates capturing relative displacement and orientation of the bodies. Time series of the coordinates from MD then yield the global structure of the DX motif and its stiffness in the harmonic approximation.