Radius of gyration of plasmid DNA isoforms from static light scattering

Despite the extensive interest in applications of plasmid DNA, there have been few direct measurements of the root mean square radius of gyration, R(G), of different plasmid isoforms over a broad range of plasmid size. Static light scattering data were obtained using supercoiled, open-circular, and linear isoforms of 5.76, 9.80, and 16.8 kbp plasmids. The results from this study extend the range of R(G) values available in the literature to plasmid sizes typically used for gene therapy and DNA vaccines. The experimental data were compared with available theoretical expressions based on the worm-like chain model, with the best-fit value of the apparent persistence length for both the linear and open-circular isoforms being statistically identical at 46 nm. A new expression was developed for the radius of gyration of the supercoiled plasmid based on a model for linear DNA using an effective contour length that is equal to a fraction of the total contour length. These results should facilitate the development of micro/nano-fluidic devices for DNA manipulation and size-based separation processes for plasmid DNA purification.