DISSOCIATION OF PROTONATED PHENYLTHIOHYDANTOIN-AMINO ACIDS AND PHENYLTHIOCARBAMOYL-DIPEPTIDES

The N-terminal phenylthiocarbamoyl (PTC) derivatives of peptides and t he phenylthiohydantoin (PTH) derivatives of amino acids are the two ma jor types of products generated in the Edman protein sequencing method . Understanding the fragmentation pathways of these species should fac ilitate structural elucidation and chemical identification based on th e fragment ion mass spectra, particularly when mass spectrometry is co mbined with the Edman sequencer for the analysis of non-standard and m odified amino acids. In this study, dissociation of the protonated PTH -X (where X = Thr, Ser, Trp and Tyr), PTH-Gly, PTC-X-Leu and PTC-Gly-L eu in electrospray ionization mass spectrometry was examined to invest igate whether there is any isomerization of PTC to PTH derivatives in the gas phase during the fragmentation. It is shown that dissociation of the protonated PTH-X proceeds via hydrogen transfer from the side-c hain of the amino acid to the PTH moiety with the elimination of the s ide-chain as a neutral species. The ions at m/z 193 formed from the so urce fragmentation of the protonated PTH-X are found to have the same structure and fragmentation pathways. The presence of this mit 193 ion and its collisionally induced dissociation (CID) spectrum are unique for the PTH derivatives and they can be used to detect the presence of the PTH ions. It is shown that there is no isomerization of the thiaz olone ions to the PTH ions during the dissociation of PTC-X-Leu tin th is case, the b(1) ions from PTC-X-Leu are believed to have the protona ted thiazolone structure). In addition, comparative studies of CID spe ctra of PTH-X and PTH-Gly or PTC-X-Leu and PTC-Gly-Leu are presented. The proposed fragmentation mechanisms for the protonated PTH and PTC d erivatives and the m/z 193 ions are given. (C) 1998 John Wiley & Sons, Ltd.