Liquid secondary ion mass spectra of 2-iminothiolane derivatives

The derivatization reaction of 2-iminothiolane with several small peptides has been studied using liquid secondary ion mass spectrometry. The 2-iminothiolane reagent reacts with side-chain amino groups and with N-terminal amines in peptides. Addition of 2-iminothiolane introduces a free sulfhydryl group and an immonium group into the peptide. The signal intensity for the derivative of a small peptide is approximately the same as that of the [M + H]+ ion of the same underivatized peptide from a glycerol solution. The pattern of dissociation observed in the product ion MS/MS spectrum is altered to reflect the derivative. The sulfhydryl group introduced via 2-iminothiolane reaction further allows for the incorporation of maleimide fluorophores into the peptide for sensitive detection by ultraviolet/visible absorption or fluorescence. This work also reports a previously unrecognized recyclization side-reaction that involves loss of the immonium group during the derivatization reaction.