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 of Flight S econdary Ion Mass Spectrometry (ToF-SIMS) was introduced to study the solid state instability of a methionine containing peptide caused by the oxidati on 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 of oxidiz ed molecular ion over unoxidized molecular ion. ME is oxidized at a faster rate to its sulfoxide derivative in the UV accelerated oxidation environmen t than in lab air. The calibration curve for evaluating the ionization prob ability ratio of the oxidized deprotonated molecular ion divided by the uno xidized deprotonated molecular ion was obtained. This could be used to extr act the real oxidation rate of ME in the solid state. Conclusions. The preliminary results showed that ToF-SIMS with simple sampl e handling, fast data acquisition, together with excellent surface sensitiv ity and detection limit could be an applicable and convenient tool to study peptide reactions in the solid state such as oxidation and deamidation pro cess.