Structures of Fluoranthene Reagent Anions Used in Electron Transfer Dissociation and Proton Transfer Reaction Tandem Mass Spectrometry

Ion/ion reactions have in recent years seen widespread use in ion activation methods such as electron transfer dissociation (ETD) tandem mass spectrometry (MS/MS) as well as in charge manipulation of highly charged peptides/proteins and their fragments by proton transfer reaction (PTR). These techniques have, in combination, enabled top-down proteomics on limited-resolution benchtop mass spectrometry platforms such as quadrupole ion traps. Anions generated by chemical ionization of fluoranthene are often used for both ETD and PTR reactions; the radical anion of fluoranthene (m/z 202) for ETD and the closed-shell anion resulting from H atom attachment to the radical anion (m/z 203) for PTR. Here we use infrared ion spectroscopy in combination with density functional theory calculations to identify the structures of these reagent anions. We establish that the m/z 203 PTR reagent anion possesses a structure that deviates from what has been suggested previously and provides some insight into the reaction mechanism involved in PTR.