Structural determination of the novel fragmentation routes of zwitteronic morphine opiate antagonists naloxonazine and naloxone hydrochlorides using electrospray ionization tandem mass spectrometry

Electrospray ionization quadrupole time‐of‐flight (ESI‐QqToF) mass spectra of the zwitteronic salts naloxonazine dihydrochloride 1 and naloxone hydrochloride 2 , a common series of morphine opiate receptor antagonists, were recorded using different declustering potentials. The singly charged ion [M+H2HCl] + at m/z 651.3170 and the doubly charged ion [M+2H2HCl] 2+ at m/z 326.1700 were noted for naloxonazine dihydrochloride 1 ; and the singly charged ion [M+HHCl] + at m/z 328.1541 was observed for naloxone hydrochloride 2 . Low‐energy collision‐induced dissociation tandem mass spectrometry (CID‐MS/MS) experiments established the fragmentation routes of these compounds. In addition to the characteristic diagnostic product ions obtained, we noticed the formation of a series of radical product ions for the zwitteronic compounds 1 and 2 , and also the formation of a distonic ion product formed from the singly charged ion [M+HHCl] + of naloxone hydrochloride 2 . Confirmation of the various established fragmentation routes was effected by conducting a series of ESI‐CID‐QqTof‐MS/MS product ion scans, which were initiated by CID in the atmospheric pressure/vacuum interface using a higher declustering potential. Deuterium labeling was also performed on the zwitteronic salts 1 and 2 , in which the hydrogen atoms of the OH and NH groups were exchanged with deuterium atoms. Low‐energy CID‐QqTof‐MS/MS product ion scans of the singly charged and doubly charged deuteriated molecules confirmed the initial fragmentation patterns proposed for the protonated molecules. Precursor ion scan analyses were also performed with a conventional quadrupole‐hexapole‐quadrupole tandem mass spectrometer and allowed the confirmation of the genesis of some diagnostic ions. Copyright © 2007 John Wiley & Sons, Ltd.