Mass spectral characterization of tetracyclines by electrospray ionization, H/D exchange, and multiple stage mass spectrometry

Abstract

Electrospray ionization (ESI) and collisionally induced dissociation (CID) mass spectra were obtained for five tetracyclines and the corresponding compounds in which the labile hydrogens were replaced by deuterium by either gas phase or liquid phase exchange. The number of labile hydrogens, x, could easily be determined from a comparison of ESI spectra obtained with N 2 and with ND 3 as the nebulizer gas. CID mass spectra were obtained for [M + H] + and [M − H] − ions and the exchanged analogs, [M(D x ) + D] + and [M(D x ) − D] −, and produced by ESI using a Sciex API-III plus and a Finnigan LCQ ion trap mass spectrometer. Compositions of product ions and mechanisms of decomposition were determined by comparison of the MS N spectra of the un-deuterated and deuterated species. Protonated tetracyclines dissociate initially by loss of H 2O (D 2O) and NH 3 (ND 3) if there is a tertiary OH at C-6. The loss of H 2O (D 2O) is the lower energy process. Tetracyclines without the tertiary OH at C-6 lose only NH 3 (ND 3) initially. MS N experiments showed easily understandable losses of HDO, HN(CH 3) 2, CH 3 − NCH 2, and CO from fragment ions. The major fragment ions do not come from cleavage reactions of the species protonated at the most basic site. Deprotonated tetracyclines had similar CID spectra, with less fragmentation than those observed for the protonated tetracyclines. The lowest energy decomposition paths for the deprotonated tetracyclines are the competitive loss of NH 3 (ND 3) or HNCO (DNCO). Product ions appear to be formed by charge remote decompositions of species de-protonated at the C-10 phenol.