The Solid State Oxidation of Methionine Containing Peptide: A Preliminary Study Using Time of Flight Secondary Ion Mass Spectrometry

Purpose. A surface sensitive mass spectrometric technique: Time ofFlight Secondary Ion Mass Spectrometry (ToF-SIMS) was introducedto study the solid state instability of a methionine containing peptidecaused by the oxidation of the methionine residue. Methods. The oxidation of a neuropeptide Methinonine-Enkephalin(ME) in air and under UV acceleration was studied by ToF-SIMS. Results. The apparent oxidation rate is defined by the peak ratio ofoxidized molecular ion over unoxidized molecular ion. ME is oxidizedat a faster rate to its sulfoxide derivative in the UV accelerated oxidationenvironment than in lab air. The calibration curve for evaluating theionization probability ratio of the oxidized deprotonated molecular iondivided by the unoxidized deprotonated molecular ion was obtained.This could be used to extract the real oxidation rate of ME in thesolid state. Conclusions. The preliminary results showed that ToF-SIMS with simplesample handling, fast data acquisition, together with excellentsurface sensitivity and detection limit could be an applicable and convenienttool to study peptide reactions in the solid state such as oxidationand deamidation process.