Differentiation of aminohydroxypropanesulfonic acid structural isomers using tandem mass spectrometry-based methods

D-Cysteinolic acid (D-CA) is an important metabolite within the biosulfur cycle. A structural isomer, (R)-3-amino-2-hydroxypropanesulfonate ((R)-AHPS), is less common in nature but potentially can be misidentified as D-CA due to their many shared physical properties. To support confident assignment of these two isomers by use of mass spectrometry alone, this study explores the fragmentation reactions of their [M + H]+ and [M -H]-ions using collision-induced dissociation (CID). Electrospray ionization mass spectrometry (ESI-MS) experiments were con-ducted on authentic standards using an ion trap mass spectrometer, while a triple-quadrupole (QqQ) mass spec-trometer was used in the selective reaction monitoring (SRM) mode to record energy-resolved CID. Density-functional theory (DFT) calculations were carried out at the M06/6-31+G* level of theory to study gas-phase fragmentation mechanisms. The data generated revealed kinetically-controlled fragmentations involving partici-pation of neighboring amino groups in the positive ion mode. Negative ion mode MS analysis could distinguish the structural isomers through different collision energy-resolved results for m/z 95 product ions, CH3SO3-. DFT cal-culations revealed an enthalpy (Delta H) (Gibbs energy (Delta G)) gap of 31.8 (31.4) kJ/mol between transition state barriers of a concerted mechanism for D-CA, and a more preferred stepwise mechanism for (R)-AHPS.