Gas phase reaction of substituted isoquinolines to carboxylic acids in ion trap and triple quadrupole mass spectrometers after electrospray ionization and collision-induced dissociation.
Within the mass spectrometric study of bisubstituted isoquinolines that possess great potential as prolylhydroxylase inhibitor drug candidates (e.g., FG-2216), unusually favored gas-phase formations of carboxylic acids after collisional activation were observed. The protonated molecule of [(1-chloro-4-hydroxy-isoquinoline-3-carbonyl)-amino]-acetic acid was dissociated, yielding the 1-chloro-4-hydroxy-isoquinoline-3-carboxylic acid methyleneamide cation. Subsequent dissociation caused the nominal elimination of 11 u that resulted from the loss of HCN and concomitant addition of oxygen to the product ion, which formed the protonated 1-chloro-4-hydroxy-isoquinoline-3-carboxylic acid. The preference of this structure under mass spectrometric conditions was substantiated by tandem mass spectrometry analyses using the corresponding methyl ester (1-chloro-4-hydroxy-isoquinoline-3-carboxylic acid methyl ester) that eliminated methylene (-14 u) upon collisional activation. Moreover, the major product ion of 1-chloro-4-hydroxy-isoquinoline-3-carboxylic acid, which resulted from the loss of water in MS3 experiments, restored the precursor ion structure by re-addition of H2O. Evidences for these phenomena were obtained by chemical synthesis of proposed gas-phase intermediates, H/D exchange experiments, high-resolution/high accuracy mass spectrometry at MSn level, and "ping-pong" analyses (MS7, in which the precursor ion was dissociated and the respective product ion isolated to regenerate the precursor ion for repeated dissociation. Based on these results, dissociation pathways for [(1-chloro-4-hydroxy-isoquinoline-3-carbonyl)-amino]-acetic acid were suggested that can be further utilized for the characterization of structurally related compounds or metabolic products in clinical, forensic, or doping control analysis.
Thevis
Mario
M
Center for Preventive Doping Research, Institute of Biochemistry, German Sport University Cologne, Cologne, Germany. m.thevis@biochem.dshs-koeln.de
Kohler
Maxie
M
Schlörer
Nils
N
Schänzer
Wilhelm
W
D002264:Carboxylic Acids / Q000737:chemistry*; D002853:Chromatography, Liquid; D005740:Gases / Q000737:chemistry*; D007546:Isoquinolines; D015394:Molecular Structure; D004364:Pharmaceutical Preparations / Q000737:chemistry; D021241:Spectrometry, Mass, Electrospray Ionization / Q000379:methods*; D053719:Tandem Mass Spectrometry
D002264:Carboxylic Acids; D005740:Gases; D007546:Isoquinolines; D004364:Pharmaceutical Preparations