Charge detection mass spectrometry (CDMS) has been used to investigate the charging of DNA plasmids of various sizes. The charge resulting from electrospray of native megadalton sized particles is thought to follow the charged residue model (CRM) where analytes accept charge from the ESI droplet representative of the size or structure of the molecule. Initial CDMS results of dsDNA plasmid pBR322, for example, show a charge distribution that spans from 600 to 1000 charges. In its supercoiled form, pBR322 is approximately 300 nm in length, which is much larger than the expected droplet size formed from the nanoESI source. When pBR322’s structure is altered into the nicked/circular or linearized form, the charge increases from that of the supercoiled plasmid. Surprisingly, we also find a low charge species, of a similar mass to the plasmid, centered at around 90 charges for pBR322. This lower charged population is attributed to a highly compact plasmid that may be compacted in the electrospray droplet. Experiments have been performed for a variety of plasmid sizes and electrospray conditions. The result shed light on the charging of very large macromolecules in electrospray.

Deciphering the charge profile of megadalton sized dsDNA plasmids with charge detection mass spectrometry

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