Dissociation of Macromolecules in Laser-Heated Droplets Monitored by CD-MS

Charge detection mass spectrometry (CD-MS) is used to monitor the dissociation of large (300 kDa to 20 MDa) protein complexes in droplets heated with a 10.6 μm CO₂ laser. In this approach, electrospray ionization (ESI) is used to produce charged droplets containing macromolecular complexes. As the droplets travel from the ESI capillary tip to the entrance of the CD-MS instrument, they pass through a variable-power laser field, where they are rapidly heated and dissociate to produce fragments. The approach is illustrated for three model systems: glutamate dehydrogenase (GDH), a 334 kDa hexameric protein complex, which dissociates into protein monomers, dimers, and tetramers; the ∼3 MDa T = 3, and ∼4 MDa T = 4 hepatitis B virus VLPs (virus-like particles) that produce a distribution of protein dimer clusters; and the ∼20 MDa T = 7 human papillomavirus VLP, which dissociates primarily into small capsid protein clusters that are not well-resolved by CD-MS. The fragments produced by in-droplet activation provide information that is useful for characterizing the structures of the intact antecedent complexes. A discussion of the advantages and current limitations of this approach is presented.