Long-term structural changes of plasmid DNA studied by atomic force microscopy

Long-term stability of plasmid DNA (pDNA) conformations is critical in many research areas, especially those concerning future gene therapy. Despite its importance, the time-evolution of pDNA structures has rarely been studied at a molecular resolution. Here, the time-evolution of pDNA solutions spanning four years was observed with atomic force microscopy (AFM). The AFM data show that the pDNA molecules changed over time from isolated supercoiled structures, to aggregated supercoiled structures, to thin, branched network structures, and finally to wider, branched network structures. Additional topographical analysis of the AFM data suggests that the actions of residual proteins could be the main mechanism for the structural changes in our laboratory-prepared pDNA.