Discrimination among the four y-hydroxyornithine diastereomers was studied by fast atom bombardment mass spectrometry (FABMS). It is impossible to distinguish among the four diastereomers of this amino acid by positive-and negative-ion FAB and collisionally activated dissociation MS, but benzyloxycarbonyl group protection of the a-and &amino groups in y-hydroxyornithine allows differentiation among the diastereomers in negative-ion FABMS. The negative-ion mass spectra of benzyloxycarbonyl-protected y-hydroxyornithine diastereomers showed differences among the abundances of the molecule ion [ M -HI -, the dehydrated ion [ M -H -H,O] -due to the loss of the y-hydroxyl group and the fragment ions formed from both [ M -HIand [ M -H -H,Ojions.
On the other hand, no difference was found between the fragmentations of the benzyloxycarbonyl-protected enantiomers of ornithine in negative-ion FABMS. These results indicate that the orientation of the y-hydroxyl group and the existence of two benzene rings in the benzyloxycarbonyl group are important factors which are responsible for the fragmentations of the four benzyloxycarbonyl-protected y-hydroxyornithine diastereomers in negative-ion FABMS. These studies also showed that the negative-ion FABMS for benzyloxycarbonyl-protected yhydroxyornithine diastereomers is a useful method for determining the configuration of each diastereomer of y-hydrox yornithine.