Dueling electrospray implemented on a traveling-wave ion mobility/time-of-flight mass spectrometer: Towards a gas-phase workbench for structural biology

Abstract

The traveling wave trap cell of a commercial ion mobility mass spectrometer (IM/MS) was used as a gas-phase reactor for covalent chemistry by making a simple modification to a standard nanoelectrospray source. Reagents and analytes were generated from pulsed opposite polarity nanoelectrospray sources and isolated by their m/z prior to reaction. Covalent bond formation was first observed with the model peptide angiotensin I. The modification site was identified as the N-terminus of the peptide by collision induced dissociation (CID). The IM cell separated the covalent reaction product from the proton transfer product by their respective ion mobilities. Next, the effects of several trapping parameters, including the trap traveling wave height, the trap RF voltage, and the trap pressure, were evaluated. Decreasing traveling wave height and increasing RF voltage and pressure increased the number of proton transfer events from apomyoglobin to reagent anions. The 6+ charge state of ubiquitin generated from nanospray under native-like conditions was covalently modified in the gas phase through ion/ion reactions. Probing the reacted protein with CID led to the assignment of lysine 29 and arginine 54 as reactive nucleophiles accessible to the reagent. IM analysis of the unmodified native-like 6+ charge state revealed that the gas-phase structure of the protein in the trap was in its compact form. Overall, we introduce a promising method for three-dimensional structural characterization of biomacromolecules.