Collision-induced dissociation of phenethylamides: role of ion-neutral complexes

Profiling aromatic constituents of Chimonanthus nitens Oliv. leaf granule using mass spectrometry

RATIONALE

The chemical constituents of Chinese patent medicine are usually different from those of crude medicine because of specific preparation processes. Chimonanthus nitens Oliv. leaf granule is widely used for prevention against COVID-19 in China. However, no research has been reported on the chemical constituents of the granule and their variation during the preparation process.

METHODS

Fragmentation patterns of reference compounds were investigated using electrospray ionization mass spectrometry, and the new gas-phase reaction was demonstrated by electronic and steric effects and calculated chemistry. Then, a strategy based on new fragmentation patterns was used to profile aromatic constituents. In addition, based on untargeted metabolomics analytical workflow, a comparison was made on the chemical constituents of the leaf and granule.

RESULTS

New fragmentation patterns related to two competing reactions, ring-opening and ring-closing reactions for coumarin, have been proposed and investigated in depth. The newly established diagnostic ion at m/z 81.0331 worked strongly in the assignment of OH-7 and substituent at C-8 of coumarin. McLafferty rearrangement occurring in coumarin glycoside while sugar group locating at C-4 was first observed, and new diagnostic ions at m/z 147.0440, 119.0488, and 91.0543 were constructed.

CONCLUSIONS

Aromatic constituents of the granule were first profiled. A total of 114 aromatic compounds were identified; of these 85 compounds were identified first. Kaempferol-7-O-neohesperidoside and its homologues were mostly enriched in the granule. Considering their reported bioactivities, these analogues possibly contribute greatly to clinical efficacy. Our results provided a new fragmentation theory for coumarins and a new material basis for the quality control of the granule.

Low Energy Collision-Induced Dissociation of Azepine Pictet-Spengler Adducts of Nω-Methylserotonin

Cimitrypazepines are a class of natural products produced by Pictet-Spengler condensation of N_ω-methylserotonin and aldehydes in a manner that produces a seven-membered azepine ring. In this study, the fragmentation behavior of this class of molecules under low-energy CID was investigated in detail. Proposed mechanisms of fragmentation were supported by deuterium labeling and DFT calculations. Loss of methylamine and methylenimine were dominant fragmentation pathways of analogs containing an aliphatic side chain. Loss of methylenimine was found to proceed via unusual methyl cation transfer. Fragmentation of analogs containing an aromatic side chain was strongly influenced by the nature of the substituents and proceeded via a novel retro-Pictet-Spengler pathway and involvement of ion-neutral complexes. In some cases, a gas-phase interconversion between the azepine and β-carboline ring was observed during fragmentation. Detailed analysis of fragmentation behavior provided in this study will serve as a valuable guide for the discovery of new analogs from natural sources.

First Direct Evidence of Interpartner Hydride/Deuteride Exchanges for Stored Sodiated Arginine/Fructose-6-phosphate Complex Anions within Salt-Solvated Structures

Mass spectrometric investigations of noncovalent binding between low molecular weight compounds revealed the existence of gas-phase (GP) noncovalent complex (NCC) ions involving zwitterionic structures. ESI MS is used to prove the formation of stable sodiated NCC anions between fructose (F6P) and arginine (R) moieties. Theoretical calculations indicate a folded solvated salt (i.e., sodiated carboxylate interacting with phosphate) rather than a charge-solvated form. Under standard CID conditions, [(F6P+R-H+Na)-H]^- competitively forms two major product ions (PIs) through partner splitting [(R-H+Na) loss] and charge-induced cross-ring cleavage while preserving the noncovalent interactions (noncovalent product ions (NCPIs)). MS/MS experiments combined with in-solution proton/deuteron exchanges (HDXs) demonstrated an unexpected labeling of PIs, i.e., a correlated D-enrichment/D-depletion. An increase in activation time up to 3000 ms favors such processes when limited to two H/D exchanges. These results are rationalized by interpartner hydride/deuteride exchanges (⟨HDX⟩) through stepwise isomerization/dissociation of sodiated NCC-d11 anions. In addition, the D-enrichment/D-depletion discrepancy is further explained by back HDX with residual water in LTQ (selective for the isotopologue NCPIs as shown by PI relaxation experiments). Each isotopologue leads to only one back HDX unlike multiple HDXs generally observed in GP. This behavior shows that NCPIs are zwitterions with charges solvated by a single water molecule, thus generating a back HDX through a relay mechanism, which quenches the charges and prevents further back HDX. By estimating back HDX impact on D-depletion, the interpartner ⟨HDX⟩ complementarity was thus illustrated. This is the first description of interpartner ⟨HDX⟩ and selective back HDX validating salt-solvated structures.

Comparing the fragmentation reactions of protonated cyclic indolyl α-amino esters in quadrupole/orbitrap and quadrupole time-of-flight mass spectrometers

RATIONALE

The comparative study of higher-energy collisional dissociation (HCD) and collision-induced dissociation (CID) mechanisms for protonated cyclic indolyl α-amino esters in quadrupole/orbitrap (Q/Orbitrap) and quadrupole time-of-flight (QTOF) mass spectrometers, respectively, is helpful to study the structures and properties of biologically active indole derivatives using tandem mass spectrometry (MS/MS) technology.

METHODS

HCD and CID experiments were carried out using electrospray ionization Q/Orbitrap MS and QTOFMS in positive ion mode, respectively. Only the labile hydrogens were exchanged with deuterium in hydrogen/deuterium exchange (HDX) experiments and only the aromatic indole C-H hydrogens were substituted with deuterium in regiospecific hydrogen-deuterium labeling experiments. Theoretical calculations were carried out using the density functional theory (DFT) method at the B3LYP level with the 6-311G(d,p) basis set in the Gaussian 03 package of programs.

RESULTS

In Q/Orbitrap MS/MS, when the added proton on the N⁸ position of protonated cyclic indolyl α-amino esters migrated in a stepwise fashion to the C³ position via two sequential 1,4-H shifts, an ion-neutral complex INC₁ of [protonated cyclic N-sulfonyl ketimino esters/indoles] was formed by a charge-directed heterolytic cleavage of the C³ -C¹⁰ bond, while an ion-neutral complex INC₃ of [cyclic N-sulfonyl ketimino esters/protonated indoles] was formed when another labile hydrogen on the N⁸ position successively migrated to the C⁴ position. Direct decomposition of INC₁ and INC₃ resulted in protonated cyclic N-sulfonyl ketimino esters and protonated indoles, respectively, while proton transfer led to protonated indoles and protonated cyclic N-sulfonyl ketimino esters. The HDX reaction with residual water in the HCD cell was also observed. In QTOF-MS/MS, protonated cyclic N-sulfonyl ketimino esters and protonated indoles resulted from direct decomposition of INC₁ and INC₃ , respectively, rather than proton transfer.

CONCLUSIONS

Due to the specific construction of the Q/Orbitrap and QTOF mass spectrometers, different fragmentation mechanisms medicated by ion-neutral complexes of protonated cyclic indolyl α-amino esters were proposed. This study is desirable for qualitative and quantitive investigation of indole derivatives using MS/MS technology.