Sulfolane-Induced Supercharging of Electrosprayed Salt Clusters: An Experimental/Computational Perspective

Abstract

It is well-known that supercharging agents (SCAs) such as sulfolane enhance the electrospray ionization (ESI) charge states of proteins, although the mechanistic origins of this effect remain contentious. Only very few studies have explored SCA effects on analytes other than proteins or peptides. This work examines how sulfolane affects electrosprayed NaI salt clusters. Such alkali metal halide clusters have played a key role for earlier ESI mechanistic studies, making them interesting targets for supercharging investigations. ESI of aqueous NaI solutions predominantly generated singly charged [Na n I(n–1)]+ clusters. The addition of sulfolane resulted in abundant doubly charged [Na n I(n–2)Sulfolane s ]2+ species. These experimental data for the first time demonstrate that electrosprayed salt clusters can undergo supercharging. Molecular dynamics (MD) simulations of aqueous ESI nanodroplets containing Na+/I– with and without sulfolane were conducted to obtain atomistic insights into the supercharging mechanism. The simulations produced [Na n I i ] z+ and [Na n I i Sulfolane s ] z+ clusters similar to those observed experimentally. The MD trajectories demonstrated that these clusters were released into the gas phase upon droplet evaporation to dryness, in line with the charged residue model. Sulfolane was found to evaporate much more slowly than water. This slow evaporation, in conjunction with the large dipole moment of sulfolane, resulted in electrostatic stabilization of the shrinking ESI droplets and the final clusters. Hence, charge–dipole stabilization causes the sulfolane-containing droplets and clusters to retain more charge, thereby providing the mechanistic foundation of salt cluster supercharging.

Controlled Formation of Nanoribbons and Their Heterostructures via Assembly of Mass‐Selected Inorganic Ions

“…We observed the entire series of HS(MoS 3 ) N 1− from N = 1–8 only when a relatively high concentration of (NH 4 ) 2 MoS 4 solution (≈10 m m ) was electrosprayed, by taking advantage of the notion that high concentration promotes cluster formation. [ 49,50 ] Out of the observed MoS ion series, only HS(MoS 3 ) 1− and HS(MoS 3 ) 3 1− have been previously observed, where HS(MoS 3 ) 1− is a typical ion observed from solutions of MoS 4 2− salts, [ 51 ] while HS(MoS 3 ) 3 1− has been observed from gas phase dissociation of HMo 3 S 13 − ions. [ 52 ] We chose to study heavy MoS ions (HS(MoS 3 ) N 1− , N ≥ 4) due to their higher adsorption energies on graphene, which allow them to adhere to graphene stronger and to be retained on graphene long enough to form the nanoribbons (Figures S1 and S2, Supporting Information).…”

“…For the electrospray ionization, (NH 4 ) 2 MoS 4 or (NH 4 ) 2 WS 4 solutions with an elevated concentration (10 mm in 1:1 water:isopropyl-alcohol) were used to promote the condensation of the Mo or W species in the generated electrospray microdroplets, as suggested by refs. [49,50] Using the ESIBD technique, described in detail elsewhere, [73] the negative ions obtained from the electrospray were characterized by using a home-built time-of-flight mass spectrometer, mass-selected using the quadrupoles, and aimed onto the surface (e.g., graphene on a TEM grid) held at room temperatures in a high vacuum (HV) chambers of 10 −7 mbar. The intact landing of the deposited MoS or WS ions was ensured by applying a voltage on the surface which would decelerate the incident ions to a low kinetic energy of ≈3 eV, considered to be well in the range of "soft landing".…”

Top-Down Analysis of Supercharged Proteins Using Collision-, Electron-, and Photon-Based Activation Methods

“…Increasing the charge states of proteins is another tactic typically enabled by addition of solution modifiers prior to ESI − resulting in the production of “supercharged” ions. Examples of commonly used supercharging agents include m -nitrobenzyl alcohol, sulfolane, and ethylene carbonate, among others. − The mechanism of supercharging is still under investigation. The use of these agents can improve mass spectrometry analyses of proteins by shifting the m / z of the ions to a less congested region, separating overlapping ions, in some cases consolidating ion current into fewer charge states or increasing fragmentation efficiency during MS/MS.…”

Top-Down Analysis of Supercharged Proteins Using Collision-, Electron-, and Photon-Based Activation Methods

“…Examples of commonly used supercharging agents include m-nitrobenzyl alcohol, sulfolane, and ethylene carbonate, among others. [45][46][47] The use of these agents can improve mass spectrometry analyses of proteins by shifting the m/z of the ions to a less congested region, separating overlapping ions, in some cases consolidating ion current into fewer charge states, and/or increasing fragmentation efficiency during MS/MS. The mechanism of supercharging is still under investigation; however, some possible explanations are based on an increase in solvent surface tension which can lead to production of more highly charged droplets, and increased solvent polarity which may stabilize charge sites.…”

“…The mechanism of supercharging is still under investigation; however, some possible explanations are based on an increase in solvent surface tension which can lead to production of more highly charged droplets, and increased solvent polarity which may stabilize charge sites. [42][43][44][45][47][48][49] Supercharging may also lead to significant changes in protein fragmentation, offering the potential to increase the information harvested from top-down MS/MS analysis. For example, one study showed that supercharging can result in the funneling of fragmentation into a few dominant backbone cleavages while reducing the abundance of fragmentation at neighboring residues.…”