Supercharging protein ions in native mass spectrometry using theta capillary nanoelectrospray ionization mass spectrometry and cyclic alkylcarbonates

Theta nanoelectrospray ionization of protein ions formed from aqueous buffer solutions that are mixed with denaturing solutions containing cyclic alkylcarbonates (e.g., vinyl ethylene carbonate; VEC) results in a significant increase in the extent of ion charging compared to native mass spectrometry. For six proteins, the extent of ion charging can be significantly higher than that obtained using denaturing solutions and alternative native “supercharging” methods. In theta nanoelectrospray supercharging, the extent of charging scales with protein mass in agreement with an analytical scaling relationship for ions with elongated structures. Theta nanoelectrospray supercharging of non-covalent complexes from native solutions results in essentially the complete loss of protein-ligand and protein-protein interactions. Based on circular dichroism spectroscopy, VEC can effectively denature proteins in buffered solutions. These data provide evidence that enrichment of VEC in theta nanoelectrospray ionization generated droplets can denature proteins on the timescale of droplet desolvation and ion formation. This approach can be used to form highly charged protein ions from native solutions containing biological buffers, including some that are considered incompatible with native MS. Forming some protein ions in the highest reported charge states directly from native solutions is no longer a challenge in obtaining primary structural information using tandem mass spectrometry.