Compaction, Relaxation, and Linearization of Megadalton-Sized DNA Plasmids: DNA Structures Probed by CD-MS.

High purity plasmid DNA is a raw material for recombinant protein production as well as an active ingredient in DNA vaccines. There are four primary plasmid structures that can be observed in a typical plasmid formulation: supercoiled, relaxed (circular), linearized, and condensed. Determining what structures are present in a sample is important, as the structure can affect activity; the supercoiled structure has the highest activity, and >90% supercoiled is desired for industry standards. Recently, charge detection mass spectrometry (CD-MS) was used to distinguish two of the structures, supercoiled and condensed, by measuring the charge deposited on the ions by positive mode electrospray. Here, CD-MS is used to probe the structures of DNA plasmids during compaction with polycations, and through enzymatic treatment to relax and linearize plasmids. We find that all four structural types for plasmid DNA have unique charging profiles that can be distinguished using CD-MS. The extent of mechanical shearing of the DNA plasmids during electrospray is strongly influenced by the structural type.