Cryogenic Ion Mobility-MassSpectrometry CapturesHydrated Ions Produced During Electrospray Ionization.

Evaporation of water from extensively hydrated protonsand peptidesformed by electrospray ionization (ESI) has been examined for thefirst time by cryogenic ion mobility-mass spectrometry (IM-MS). Theextent of hydration was controlled using a heated capillary inletoperated between 340 and 391 K. Cold cluster ions formed in the sourceregion were transported into a low temperature (∼80 K) IM drifttube using an electrostatic ion guide where they were separated onthe basis of size-to-charge via low-energy collisions with heliumgas. The eluting IM profile was subsequently pulsed into an orthogonaltime-of-flight (TOF) mass spectrometer for mass-to-charge (m/z) identification of the cluster ionspecies. Key parameters that influence the cluster distributions werecritically examined including the inlet temperature, drift tube temperature,and IM field strength. In agreement with previous studies, our findingsindicate that water evaporation is largely dependent upon the particularcharge-carrying species within the cluster. IM-MS results for protonatedwater clusters suggest that the special stability of H+(H2O)n(n= 21) is attributed to the presence of a compact isomer (assignedto a clathrate cage) that falls below the trendline produced by adjacentclusters in the n= 15 to 35 size range. Peptidestudies are also presented in which specific and nonspecific solvationis observed for gramicidin S [GS + 2H]2+(H2O)n(n= 0 to ∼26) andbradykinin [BK + 2H]2+(H2O)n(n= 0 to ∼73), respectively. [ABSTRACT FROM AUTHOR]