Protein charge-state distributions in electrospray-ionization mass spectrometry do not appear to be limited by the surface tension of the solvent

According to a current model for protein electrospray, the charge-state distributions (CSDs) observed by electrospray-ionization mass spectrometry (ESI-MS) are controlled by the Rayleigh-limit charge of the droplets that generate the gas-phase protein ions. A testable prediction of this model is that the maximum charge state displayed by proteins in ESI-MS should respond to changes in the surface tension of the ESI droplets according to the Rayleigh equation. In this work, we subject this specific hypothesis to direct experimental testing. We show data obtained by time-of-flight (TOF) nano-ESI-MS with several different proteins in aqueous solutions containing 20-50% 1-propanol or 40% 1,2-propylene glycol. Both of these compounds have lower vapor pressure and lower surface tension than water. Propylene glycol also has a lower evaporation rate than water, providing an even more stringent test for surface tension effects in late ESI droplets. None of these cosolvents affects the CSDs of either folded or unfolded proteins as predicted by the Rayleigh-charge model. The only changes induced by 1-propanol can be ascribed to protein unfolding triggered above critical concentrations of the alcohol. Below such a threshold, no shift of the CSDs toward lower charge states is observed. The presence of 40% propylene glycol in the original protein solutions gives rise to CSDs that either are the same as those in the control samples or present much smaller changes than those calculated by the Rayleigh equation. Thus, the charge states of gas-phase protein ions produced by electrospray do not seem to be limited by the surface tension of the solvent. They rather appear to be quite protein-specific.