Field-free capillary vibrating sharp-edge spray ionization
(cVSSI)
is evaluated for its ability to conduct native mass spectrometry (MS)
experiments. The charge state distributions for nine globular proteins
are compared using field-free cVSSI, field-enabled cVSSI, and electrospray
ionization (ESI). In general, for both positive and negative ion mode,
the average charge state (q
avg) increases
for field-free cVSSI with increasing molecular weight similar to ESI.
A clear difference is that the q
avg is
significantly lower for field-free conditions in both analyses. Two
proteins, leptin and thioredoxin, exhibit bimodal charge state distributions
(CSDs) upon the application of voltage in positive ion mode; only
a monomodal distribution is observed for field-free conditions. In
negative ion mode, thioredoxin exhibits a multimodal CSD upon the
addition of voltage to cVSSI. Extensive molecular dynamics (MD) simulations
of myoglobin and leptin in nanodroplets suggest that the multimodal
CSD for leptin may originate from increased conformational “breathing”
(decreased packing) and association with the droplet surface. These
properties along with increased droplet charge appear to play critical
roles in shifting ionization processes for some proteins. Further
exploration and development of field-free cVSSI as a new ionization
source for native MS especially as applied to more flexible biomolecular
species is warranted.