Intriguing Differences in the Gas-Phase Dissociation Behavior of Protonated and Deprotonated Gonyautoxin Epimers

The aim of this study was to investigate the unusual gas-phase dissociation behavior of two epimer pairs of protonated gonyautoxins (GTX) following electrospray ionization in comparison to their deprotonated counterparts. The chemical structures of the investigated GTX1-4 variants vary in their substitution pattern at N-1 and the stereochemical orientation of the hydroxysulfate group at C-11 (11 alpha for GTX1/2 versus 11 beta for GTX3/4). The direct comparison of mass spectra in positive and negative ion modes illustrated two distinct features: first, an intriguing difference between protonated 11 alpha and 11 beta species, where 11 alpha conformations exhibited almost complete dissociation of [M + H](+) ions via facile SO3 elimination, while 11 beta species remained mostly intact as [M + H](+); and second, the lack of such differences for the deprotonated counterparts. In this study, we propose an acid-catalyzed elimination mechanism from density functional theory calculations, initiated by a proton transfer of a guanidinium proton to the hydroxysulfate group with simultaneous SO3 release, which is only possible for the 11 alpha conformation based on intramolecular distances. The same mechanism explains the lack of a comparable SO3 loss in the negative ion mode. CID experiments supported this proposed mechanism for GTX1 and GTX2. Computational modeling of product ions seen in the CID spectra of GTX3 and GTX4 established that the lowest energy dissociation pathway for the 11 beta epimers is elimination of water with the possibility for further SO3 release from the intermediate product. Experimental data for structurally analogous decarbamoyl gonyautoxins confirmed the evidence for the GTX compounds as well as the proposed elimination mechanisms.