Mass spectra of cysteine derivates

The mass spectra of a series of cysteine derivatives of structure X-CH2CH2SCH2CH(NH2)COOH were examined to assess the influence of the electron-withdrawing functional group X on the mass spectral fragmentation patterns. Measurable molecular ions were present in most of the spectra although in some cases such peaks had relative abundances below a few per cent. More useful information on the nature of the substituent could be obtained from the M - 74 peak corresponding to cleavage C2H4O2N at the sulfur atom. The results show that mass spectroscopy is valuable for identifying the S-alkyl side chain in S-alkyl cysteine derivatives, a process frequently required in studies on chemical modification of sulfhydryl groups andin determining disulfide bonds in proteins. The observed fragmentation patterns are discussed in terms of localization of positive charges of ionic species on either sulful, nitrogen, or heterocyclic rings and in terms of substituent effects on available decomposition pathways.