Ion chemistry of deprotonated phenylthiocarbamyl-phenylalanine

The fragmentation of an [M-H](-) anion of phenylthiocarbamyl (PTC)-phenylalanine (Phe) and of a deuterium labeled analog using electrospray ionization (ESI) in a negative ion mode was studied. Product ion experiments show thar deprotonated PTC-Phe fragments mainly through three different pathways. Further tandem mass spectrometry experiments show that these three pathways actually correspond to the three different deprotonation sites in the molecule. These three ionization sites Lead to the losses of, respectively, H2S and (H2S + CO2), C6H5N=C=S, and C6H5NH2 from [M-H](-) in the first steps. They yield anions stabilized by resonance. The fragment produced by the loss of C6H5NH2 is relatively weak and originates from a less acidic site. Dy selecting these fragment anions successively as precursors from the ion source, detailed information on the fragmentation mechanism is obtained. The benzyl-type anion plays a stabilizing role through conjugation. The fragmentation process also involves some uncommon neutral losses. For example, losses of HCN involve reactions that may occur through four-or five-membered cyclic transition states and cyanide ion-molecule complexes. Similar losses of HCN may occur through both the carboxylate anion and one of its tautomeric forms. Deuterium labeling studies support all the mechanistic proposals. The fragments obtained following the loss of the derivatization reagent (C6H5-N=C=S) show that it is the anion of the deprotonated free amino acid. Preliminary results have shown that this also occurs for other PTC amino acids. (C) 1997 American Society for Mass Spectrometry