Leaving Group Effects in a Series of Electrosprayed CcHhN1 Anthracene Derivatives.

We investigate the gas-phase structures and fragmentation pathways of model compounds of anthracene derivatives of the general formula CcHhN1 utilizing tandem mass spectrometry and computational methods. We vary the substituent alkyl chain length, composition, and degree of branching. We find substantial experimental and theoretical differences between the linear and branched congeners in terms of fragmentation thresholds, available pathways, and distribution of products. Our calculations predict that the linear substituents initially isomerize to form lower energy branched isomers prior to loss of the alkyl substituents as alkenes. The rate-determining chemistry underlying these related processes is dominated by the ability to stabilize the alkene loss transition structures. This task is more effectively undertaken by branched substituents. Consequently, analyte lability systematically increased with degree of branching (linear < secondary < tertiary). The resulting anthracen-9-ylmethaniminium ion generated from these alkene loss reactions undergoes rate-limiting proton transfer to enable expulsion of either hydrogen cyanide or CNH. The combination of the differences in primary fragmentation thresholds and degree of radical-based fragmentation processes provide a potential means of distinguishing compounds that contain branched alkyl chain substituents from those with linear ones.

Fragmentation Pathways of Lithiated Hexose Monosaccharides.

We characterize the primary fragmentation reactions of three isomeric lithiated D-hexose sugars (glucose, galactose, and mannose) utilizing tandem mass spectrometry, regiospecific labeling, and theory. We provide evidence that these three isomers populate similar fragmentation pathways to produce the abundant cross-ring cleavage peaks (0,2A1 and 0,3A1). These pathways are highly consistent with the prior literature (Hofmeister et al. J. Am. Chem. Soc. 113, 5964-5970, 1991, Bythell et al. J. Am. Soc. Mass Spectrom. 28, 688-703, 2017, Rabus et al. Phys. Chem. Chem. Phys. 19, 25643-25652, 2017) and the present labeling data. However, the structure-specific energetics and rate-determining steps of these reactions differ as a function of precursor sugar and anomeric configuration. The lowest energy water loss pathways involve loss of the anomeric oxygen to furnish B1 ions. For glucose and galactose, the lithiated α-anomers generate ketone structures at C2 in a concerted reaction involving a 1,2-migration of the C2-H to the anomeric carbon (C1). In contrast, the ß-anomers are predicted to form 1,3-anhydroglucose/galactose B1 ion structures. Initiation of the water loss reactions from each anomeric configuration requires distinct reactive conformers, resulting in different product ion structures. Inversion of the stereochemistry at C2 has marked consequences. Both lithiated mannose forms expel water to form 1,2-anhydromannose B1 ions with the newly formed epoxide group above the ring. Additionally, provided water loss is not instantaneous, the α-anomer can also isomerize to generate a ketone structure at C2 in a concerted reaction involving a 1,2-migration of the C2-H to C1. This product is indistinguishable to that from α-glucose. The energetics and interplay of these pathways are discussed. Graphical Abstract ᅟ.