Collisionally activated dissociations of aminocyclitol-aminoglycoside antibiotics and their application in the identification of a new compound in tobramycin samples

Several aminocyclitol-aminoglycoside antibiotics have been studied by tande m mass spectrometry. Glycosidic bond cleavages were the major reactions in the low energy collisionally activated decomposition (CAD) of the protonate d antibiotics. Only the glycoside residing on the C-6-O of the 2-deoxystrep tamine was observed to undergo significant decomposition at the C-2-C-3 and O-C-1 bonds. The comprehension of the CAD of known aminoglycosides aided i n the identification of an unknown impurity in tobramycin. The unknown comp ound was initially detected by reverse phase high-performance liquid chroma tography following dinitrofluorobenzene derivatization of the amino groups. The molecular weight of the dinitrobenzene derivative measured by LC mass spectrometry (MS) led to the detection of two isomeric impurities in tobram ycin by LC-MS using an amino column. Their CAD spectra were subsequently ac quired by LC-MS/MS. One of the two compounds was determined to be a known c ompound, 6 "-O-carbamyltobramycin with the carbamyl group substituted on th e glycoside residing on the C-6-O of 2-deoxystreptamine. The fragmentation pattern of the other compound was interpreted as that the unknown was also a carbamyltobramycin. The carbamyl group was, however, substituted on 2-deo xystreptamine. It was speculated that the carbamyl group was substituted at the C-1 amino group. This compound, to our knowledge, has not been reporte d before. (C) 2000 American Society for Mass Spectrometry.