Metal complexation reactions of quinolone antibiotics in a quadrupole ion trap.

We have undertaken a systematic study of the nature of quinolone metal complexes formed by electrospray ionization and laser desorption/ion-molecule reactions to evaluate the analytical utility of metal complexation as an alternative to conventional ionization via protonation. Metal ionization with laser-desorbed copper and nickel ions results in addition products of the form (L + Cu+) and (L + Ni+), respectively, where L is the quinolone, whereas addition-elimination products of the form (L + Co(+)-28) are observed when cobalt is used. The elimination of CO in order to form this unusual latter product seems to be favored by the formation of a cyclized structure that is stabilized by intramolecular hydrogen bonding. The CAD patterns of the Ni+ complexes prove to be the most structurally informative, more so than the fragmentation patterns of the protonated quinolones. Quinolone-metal complexes of the type [MII(L-H+)-(dipy)]+, where M is either Cu, Co, or Ni and dipy is 2,2'-dipyridine, are generated by electrospray ionization of a methanolic solution containing a quinolone antibiotic, a transition metal ion salt, and an auxiliary diimine ligand. Upon collisional activation, the ESI-generated complexes dissociate predominantly by loss of CO2, which is also the most common fragmentation pathway for the metal complexes formed through laser desorption/ion-molecule reactions. However, there are fewer structurally diagnostic fragment ions in the CAD spectra of the ESI complexes relative to those of the LD complexes.