METASTABLE FRAGMENTATION OF SOMATOSTATIN-14 (SS-14) AND A SERIES OF SS-14 ANALOGS FORMED WITH LIQUID SECONDARY ION MASS-SPECTROMETRY - OBSERVATION OF FRAGMENT IONS WHICH INVOLVE UNSYMMETRIC DISULFIDE BRIDGE CLEAVAGE CONCOMITANT WITH PEPTIDE-CHAIN CLEAVAGE

Somatostatin-14 (SS-14) and several SS-14 analogs were analyzed using liquid secondary ion mass spectrometry (LSIMS). The observed isotope distributions showed low levels of [H-2-SS-14] (reduced SS-14). The daughter-ion spectra of the protonated molecule ions of SS-14 and several SS-14 analogs contained a number of metastable fragment ions. Two fragments in these spectra were assigned to cleavage of the peptide chain concomitant with unsymmetric cleavage of the disulfide bridge. Single alanine-substituted analogs of SS-14 were used to confirm these assignments, while single D isomer-substituted analogs of SS-14 were used to investigate the dependence of the cleavages on conformation.