THE DIAGNOSTIC-VALUE OF THE M Z 102 PEAK IN THE POSITIVE-ION FAST-ATOM-BOMBARDMENT MASS-SPECTRA OF PEPTIDES/

The mass spectrometric characteristics of a number of different m/z 10 2 ions, which might occur in the fast-atom bombardment mass spectra of peptides, appeared to be sufficiently different for an unequivocal st ructural assignment and they have successfully been applied in the str uctural analysis of an unknown. Special attention has been paid to the structure of m/z 102 ions from threonine-containing compounds. Attemp ts to generate m/z 102 product ions from peptides containing threonine at the N-terminus were not successful. From experiments with O-18-lab elled threonine it was concluded that the side-chain hydroxyl group is exclusively involved in the water loss from protonated threonine. Bas ed on ab initio calculations of feasible m/z 102 production structures , N-protonated dehydroamino-2-butyric acid from protonated threonine a nd peptides containing threonine at the C-terminus seems to be the mos t likely product-ion structure. Although relative stability calculatio ns revealed that the threonine B-1-type ion has the lowest heat of for mation, it could also be concluded that a B-1-type ion from threonine should be considered as an electrostatically bound ion/molecule comple x, which will immediately dissociate into a threonine immonium ion (A( 1)) and a carbon monoxide molecule. The energy requirement for this fr agmentation reaction (B-1-->A(1)+CO) is less than 7 kJ/mol without an energy barrier, which makes B-1-type ions of threonine unlikely to exi st in the gas phase.