DECOMPOSITIONS OF CATIONIZED HETERODIMERS OF AMINO-ACIDS IN RELATION TO CHARGE LOCATION IN PEPTIDE IONS

The unimolecular decompositions of protonated heterodimers of native a nd derivatized amino acids to yield the protonated monomers were studi ed as a guide to charge location in peptide ions. Analyses using a hyb rid instrument of BEqQ geometry demonstrated the advantages (with resp ect to mass resolution, sensitivity, reproducibility, and the eliminat ion of extraneous signals) of the detection of product ions formed in the radiofrequency-only quadrupole region (q) rather than in the field -free region between B and E. Conversion of arginine to dimethylpyrimi dylornithine (DMPO) reduced the proton affinity, as evidenced by the d ecomposition of the protonated arginine/DMPO heterodimer. Conversion o f cysteine to pyridylethylcysteine enhanced the proton affinity. Appli cation of these derivatization procedures to peptides resulted in chan ges in the observed fragmentations of the protonated precursors consis tent with the predicted modifications in charge location. Unimolecular decomposition of the protonated dimer composed of glycine and N-acety lglycine yielded both protonated monomers with abundances differing by a factor of only 2; this suggests that in protonated peptides, the am ide bonds are competitive with the N-terminal amino group as sites of protonation. It is clear that the propensities to proton or metal-cati on location at particular sites in peptides are influenced by both sho rt- and long-range intraionic interactions. In peptides composed of am ino acids of similar cation affinities, it may be postulated that the ion population is heterogeneous with respect to the site of charge, wi th consequent promotion of multiple low-energy fragmentation routes.