posted on 2015-01-20, 00:00authored byJ. Corinne DeMuth, Scott A. McLuckey
The exposure of aqueous nanoelectrospray
droplets to various organic
vapors can dramatically reduce sodium adduction on protein ions in
positive ion mass spectra. Volatile alcohols, such as methanol, ethanol,
and isopropanol lead to a significant reduction in sodium ion adduction
but are not as effective as acetonitrile, acetone, and ethyl acetate.
Organic vapor exposure in the negative ion mode, on the other hand,
has essentially no effect on alkali ion adduction. Evidence is presented
to suggest that the mechanism by which organic vapor exposure reduces
alkali ion adduction in the positive mode involves the depletion of
alkali metal ions via ion evaporation of metal ions solvated with
organic molecules. The early generation of metal/organic cluster ions
during the droplet desolvation process results in fewer metal ions
available to condense on the protein ions formed via the charged residue
mechanism. These effects are demonstrated with holomyoglobin ions
to illustrate that the metal ion reduction takes place without detectable
protein denaturation, which might be revealed by heme loss or an increase
in charge state distribution. No evidence is observed for denaturation
with exposure to any of the organic vapors evaluated in this work.