Imine or Schiff base formation is considered as a key event in the catalytic mechanisms of many enzymes and in certain biological transformations, including glycation. In this process, less stable amino-acid-derived Schiff bases rearrange into more stable ketoamines or Amadori products. Schiff bases are also stipulated to be stabilized through complexation with metal ions, or through intramolecular cyclization to form more stable and reversible cyclic isomers, such as oxazolidin-5-ones. These intermediates can be easily detected relative to Schiff bases due to their higher stability. In this study, high-resolution mass spectrometry and isotope labeling techniques were used to identify labile imines as their oxazolidin-5-one derivatives in heated reaction systems of glucose/alanine/FeCl2, including their 13C-labeled counterparts. The reaction mixtures were heated for 2h at 110 °C and were analyzed by high resolution qTOF/MS for the presence of masses corresponding to Schiff bases of α-alanine with short chain aldehydes that can be generated from glucose degradation and also for the incorporation of 13C-labeled atoms from 13C-3 alanine and 13C-U glucose. Analysis of the data has indicated that Schiff bases can indeed be detected in the form of oxazolidin-3-ones, when methanol is used as the solvent. Furthermore, it was discovered that metal-ion-stabilized Schiff bases, in addition to forming oxazolidin-3-ones, can also undergo aldol addition with short chain sugars and initiate oligomerization reactions, leading to the formation of dimeric or trimeric oxazolidin-3-one oligomers, as demonstrated by their characteristic MS/MS fragmentations.

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