Combining Chemical Knowledge and Quantum Calculation for Interpreting Low-Energy Product Ion Spectra of Metabolite Adduct Ions: Sodiated Diterpene Diester Species as a Case Study

Enantioselective Reduction of Noncovalent Complexes of Amino Acids with CuII via Resonant Collision-Induced Dissociation: Collision Energy, Activation Duration Effects, and RRKM Modeling

Formation of noncovalent complexes is one of the approaches to perform chiral analysis with mass spectrometry. Enantiomeric distinction of amino acids (AAs) based on the relative rate constants of competitive fragmentations of quaternary copper complexes is an efficient method for chiral differentiation. Here, we studied the complex [CuII,(Phe,PhG,Pro-H)]+ (m/z 493) under resonant collision-induced dissociation conditions while varying the activation time. The precursor ion can yield two main fragments through the loss of the non-natural AA phenylglycine (PhG): the expected product ion [CuII,(Phe,Pro-H)]+ (m/z 342) and the reduced product ion [CuI,(Phe,Pro)]+ (m/z 343). Enantioselective reduction describes the difference in relative abundance of these ions, which depends on the chirality of the precursor ion: the formation of the reduced ion m/z 343 is favored in homochiral complexes (DDD) compared to heterochiral complexes (such as LDD). Energy-resolved mass spectrometry data show that reduction, which arises from rearrangement, is favored at a low collision energy (CE) and long activation time (ActT), whereas direct cleavage preferentially occurs at a high CE and short ActT. These results were confirmed with kinetic modeling based on RRKM theory. For this modeling, it was necessary to set a pre-exponential factor as a reference, so that the E0 values obtained are relative values. Interestingly, these simulations showed that the critical energy E0 required to form the reduced ion is comparable in both homochiral and heterochiral complexes. However, the formation of product ion m/z 342 through direct cleavage is associated with a lower E0 in heterochiral complexes. Consequently, enantioselectivity would not be caused by enhanced reduction in homochiral complexes but rather by direct cleavage being favored in heterochiral complexes.

Predicting Preferences for Adduct Formation in Electrospray Ionization: The Case Study of Succinic Acid

A simple theoretical approach is developed that can be used to predict the preference of ion adduct formation (with alkali Li⁺, Na⁺, K⁺ and alkaline earth Ca²⁺, Mg²⁺ metals) in electrospray ionization mass spectrometry (ESI-MS) of succinic acid, associated with several protonation/deprotonation equilibria. The applied strategy consists of using a vacuum environment as well as both implicit and explicit solvation of reactive sites and density functional theory as the method of choice. These distinct levels of theory mimic the smooth transition between the condensed environment and free ion in the gas phase. Good correlation between the Gibbs free energies for protonation/adduct formation processes with peak observation in the obtained mass spectra provide insight into the physical basis behind adduct preference and selectivity. This signifies the relationship between microscopic interactions, ionization efficiency, and types of ions that reach the detector.

Energy-Resolved Ion Mobility Spectrometry: Composite Breakdown Curves for Distinguishing Isomeric Product Ions

Identification of lipopeptides (LpAA) synthesized from bacteria involves the study of structural characterization. Twenty LpAA have been characterized using commercial tandem high-resolution mass spectrometers in negative electrospray, employing nonresonant excitation in "RF only" collision cells and generally behave identically. However, [LpAA-H]^- (AA = Asp or Glu) shows surprising fragmentation pathways, yielding a complementary fatty acid carboxylate and dehydrated amino acid fragment anions. In this study, the dissociation mechanisms of [C12Glu-H]^- were determinate using energy-resolved mass spectrometry (ERMS). Product ion breakdown profiles are, generally, unimodal with full width at half-maximum (fwhm) increasing as product ion m/z ratios decrease, except for the two product ions of interest (fatty acid carboxylate and dehydrated glutamate) characterized by broad and composite profiles. Such behavior was already shown for other ions using a custom-built guided ion beam mass spectrometer. In this study, we investigate the meaning of these particular profiles from an ERMS breakdown, using fragmentation mechanisms depending on the collision energy. ERMS on line with ion mobility spectrometry (IMS), here called ER-IMS, provides a way to probe such questions. Broad or composite profiles imply that the corresponding product ions may be generated by two (or more) pathways, resulting in common or isomeric product ion structures. ER-IMS analysis indicates that the fatty acid carboxylate product ion is produced with a common structure through different pathways, while dehydrated glutamate has two isomeric forms depending on the mechanism involved. Drift time values correlate with the calculated collision cross section that confirms the product ion structures and fragmentation mechanisms.

Identification of bacterial lipo-amino acids: origin of regenerated fatty acid carboxylate from dissociation of lipo-glutamate anion

The identification of bacterial metabolites produced by the microbiota is a key point to understand its role in human health. Among them, lipo-amino acids (LpAA), which are able to cross the epithelial barrier and to act on the host, are poorly identified. Structural elucidation of few of them was performed by high-resolution tandem mass spectrometry based on electrospray combined with selective ion dissociations reach by collision-induced dissociation (CID). The negative ions were used for their advantages of yielding only few fragment ions sufficient to specify each part of LpAA with sensitivity. To find specific processes that help structural assignment, the negative ion dissociations have been scrutinized for an LpAA: the N-palmitoyl acyl group linked to glutamic acid (C16Glu). The singular behavior of [C16Glu-H]¯ towards CID showed tenth product ions, eight were described by expected fragment ions. In contrast, instead of the expected product ions due to CONH-CH bond cleavage, an abundant complementary dehydrated glutamic acid and fatty acid anion pair were observed. Specific to glutamic moiety, they were formed by a stepwise dissociation via molecular isomerization through ion–dipole formation prior to dissociation. This complex dissociated by partner splitting either directly or after inter-partner proton transfer. By this pathway, surprising regeneration of deprotonated fatty acid takes place. Such regeneration is comparable to that occurred from dissociation to peptides containing acid amino-acid. Modeling allow to confirm the proposed mechanisms explaining the unexpected behavior of this glutamate conjugate.