Testing the role of solvent surface tension in protein ionization by electrospray

This work was aimed at probing the influence of solvent surface tension on protein ionization by electrospray. In particular, we were interested in testing the previously suggested hypothesis that the charge-state distributions (CSDs) of proteins in electrospray ionization mass spectrometry (ESI-MS) are controlled by the surface tension of the least volatile solvent component. In the attempt to minimize uncontrolled conformational effects, we used acid-sensitive proteins (cytochrome c and myoglobin) at low pH or highly stable proteins (ubiquitin and lysozyme) in the presence of low concentrations of organic solvents. A first set of experiments compared the effect of 1- and 2-propanol. These two alcohols have similar chemico-physical properties but values of vapor pressure below and above that of water, respectively. Both compounds have much lower surface tension than water. The solvents employed allowed testing of the influence of surface tension on protein spectra obtained from similarly denaturing solutions. The compared solvent conditions gave rise to very similar spectra for each tested protein. We then investigated the effect of the addition of dimethyl sulfoxide to acid-unfolded proteins. We observed enhanced ionization in the presence of acetic or formic acid, consistent with the previously described supercharging effect, but almost no shift of the CSD in the presence of HCl. Finally, we analyzed thermally denatured cytochrome c, to obtain reference spectra of the unfolded protein in high-surface-tension solutions. Also in this case, the CSD of the unfolded protein was shifted towards lower m/z values relative to low-surface-tension systems. In contrast to the other results reported here, this effect is consistent with an influence of solvent surface tension on CSD. The magnitude of the effect, however, is much smaller than predicted by the Rayleigh equation. The results presented here are not easy to reconcile with the hypothesis that the maximum charge state exhibited by proteins in ESI-MS reflects the Rayleigh-limit charge of the precursor droplet. The data are discussed with reference to models for the mechanism of electrospray ionization.