A- to B-Form Transition in DNA Between Gold Surfaces

Molecular dynamics simulations have been performed to characterize the conformation of DNA that is present when DNA links gold nanoparticles to form nanoparticle superlattice crystals. To model the DNA-linked gold nanoparticles, four strands of DNA are used to connect two gold surfaces, with a small interstrand separation and high added salt to match experiment. A-form DNA was assumed for the initial conformation, as this form of DNA has a length per base-pair that matches lengths that have been inferred from Xray measurements. The DNA structure was monitored for 40 ns, and the distributions of the slide and z(p) coordinates were obtained from the simulations. We find that all the double-stranded DNA (ds-DNA) strands transform from A- to B-DNA during the simulations. In addition, single-stranded DNAa (ss-DNAs) that are used to connect the ds-DNA to each surface are found to become adsorbed on the gold surfaces during this process, and the ds-DNAs bend (similar to 143 degrees) at their junctions with the two gold surfaces to accommodate the observed distance between gold surfaces using B-form DNA. We infer from this that the short length of DNA between the gold surfaces is not due to the presence of A-DNA.