Fragmentation reactions of singly and doubly protonated thiourea- and sugar-substituted cyclams and their transition-metal complexes

Dereplication of Acetogenins from Annona muricata by Combining Tandem Mass Spectrometry after Lithium and Copper Postcolumn Cationization and Molecular Networks

Annonaceous acetogenins are natural products held responsible for atypical Parkinsonism due to chronic consumption in traditional medicine or as food, leading to the development of analytical strategies for their complete chemical characterization in complex mixtures. Characterization by tandem mass spectrometry (MS/MS) of acetogenins using collision-induced dissociation from lithium adducts provides additional structural information compared to protonated or sodiated species such as ketone location on the acetogenin backbone. However, very low intensity diagnostic ions together with the lack of extensive structural information regarding position of OH and THF substituents limit this approach. Copper adducts led to diagnostic fragment ions that allow us to identify the position of oxygen rings and hydroxyl substituents. Fragmentation rules were established on the basis of acetogenin standards allowing the identification of 45 over the 77 analogues observed in an extract of Annona muricata by LC-MS/MS using postcolumn infusion of copper sulfate (CuSO₄) solution. Molecular networks that were generated thanks to specific fragmentations obtained with copper led to the distinction of THF ring position or to the identification of hydroxylated lactone, for instance.

First Steps to Rationalize Host-Guest Interaction between α-, β-, and γ-Cyclodextrin and Divalent First-Row Transition and Post-transition Metals (Subgroups VIIB, VIIIB, and IIB)

Cyclodextrins (CDs) are cyclic oligosaccharides mainly composed of six, seven, and eight glucose units, so-called α-, β-, and γ-CDs, respectively. They own a very particular molecular structure exhibiting hydrophilic features thanks to primary and secondary rims and delimiting a hydrophobic internal cavity. The latter can encapsulate organic compounds, but the former can form supramolecular complexes by hydrogen-bonding or electrostatic interactions. CDs have been used in catalytic processes to increase mass transfer in aqueous-organic two-phase systems or to prepare catalysts. In the last case, interaction between CDs and metal salts was considered to be a key point in obtaining highly active catalysts. Up to now, no work was reported on the investigation of factors affecting the binding of metal to CD. In the study herein, we present the favorable combination of electrospray ionization coupled to mass spectrometry [ESI-MS(/MS)] and density functional theory molecular modeling [B3LYP/Def2-SV(P)] to delineate some determinants governing the coordination of first-row divalent transition metals (Mn²⁺, Co²⁺, Ni²⁺, Cu²⁺, and Fe²⁺) and one post-transition metal (Zn²⁺) with α-, β-, and γ-CDs. A large set of features concerning the metal itself (ionic radius, electron configuration, and spin state) as well as the complexes formed (the most stable conformer, relative abundance in MS, CE₅₀ value in MS/MS, binding energy, effective coordination number, average bond lengths, binding site localization, bond dissociation energies, and natural bond orbital distribution) were screened. Taking into account all of these properties, various selectivity rankings have been delineated, portraying differential association/dissociation behaviors. Nonetheless, unique 3D topologies for each CD-metal complex were emphasized. The combination of these approaches brings a stone for building a compendium of molecular features to serve as a suitable descriptor or predictor for a better first round rationalization of catalytic activities.

Metal-ion-assisted structural and anomeric analysis of Amadori compounds by electrospray ionization mass spectrometry

RATIONALE

The Maillard reaction plays an important role in food, physiology and traditional Chinese medicine, and its primary reaction products are formed through Amadori rearrangement by reducing sugars and amino acids. The analysis of the characteristic fragmentation and of the glycosidic bond configuration of Amadori compounds will promote their fast discovery and identification by mass spectrometry.

METHODS

Four Amadori compounds that reduce disaccharides and proline/tryptophan were used to investigate the fragmentation mechanisms via tandem mass spectrometry (MS/MS) with different alkali metal ion adducts. Cu²⁺ could be used to distinguish glycosidic bond configurations of the reducing disaccharides in the full-scan mass spectra. Quantum calculations were also conducted for a single Amadori compound with Cu²⁺ for analysis of the most optimized configurations and binding energies of metal complexes.

RESULTS

MS/MS analysis of Amadori-alkali metal complexes revealed that the radius of the alkali metal ions had profound effects on the degree of fragmentation of such compounds, among which lithium-cationized ions produced the most extensive fragmentation. Amadori compounds with different glycosidic bonds formed differently proportioned metal complexes with Cu²⁺ , and the complexity of the copper complexes containing tryptophan moieties was higher than that of those containing proline moieties in the mass spectra. Quantum calculations showed that Amadori compounds with β-configurations can form more binding sites with Cu²⁺ than those with α-configurations, thus making the metal complex with a single ligand more stable. In addition, the chelation of tryptophan with copper ions increased the coordination binding energy, which showed that α-configured Amadori compounds were readily able to form multi-ligand copper complexes.

CONCLUSIONS

Metal-ion-assisted analysis provides crucial information for structural and anomeric analysis of Amadori compounds by electrospray ionization mass spectrometry. Elucidation of binding sites and binding energies by quantum calculations has significantly improved the knowledge of metal complexes in the gas phase and provides background information for determining the glycosidic configuration of Amadori isomers.

The transformation from 2°-amine to 3°-amine of cyclam ring alters the fragmentation patterns of 1-tosylcytosine-cyclam conjugates

The novel N-1-sulfonylcytosine-cyclam conjugates 1 and 2 conjugates are ionized by electrospray ionization mass spectrometry (ESI MS) in positive and negative modes (ES⁺ and ES^- ) as singly protonated/deprotonated species or as singly or doubly charged metal complexes. Their structure and fragmentation behavior is examined by collision induced experiments. It was observed that the structure of the conjugate dictated the mode of the ionization: 1 was analyzed in ES^- mode while 2 in positive mode. Complexation with metal ions did not have the influence on the ionization mode. Zn²⁺ and Cu²⁺ complexes with ligand 1 followed the similar fragmentation pattern in negative ionization mode. The transformation from 2°-amine in 1 to 3°-amine of cyclam ring in 2 leads to the different fragmentation patterns due to the modification of the protonation priority which changed the fragmentation channels within the conjugate itself. Cu²⁺ ions formed complexes practically immediately, and the priority had the cyclam portion of the ligand 2. The structure of the formed Zn²⁺ complexes with ligand 2 depended on the number of 3° amines within the cyclam portion of the conjugate and the ratio of the metal:ligand used. The cleavage of the cyclam ring of metal complexes is driven by the formation of the fragment that suited the coordinating demand of the metal ions and the collision energy applied. Finally, it was shown that the structure of the cyclam conjugate dictates the fragmentation reactions and not the metal ions.

Deviant mass shift of hydrated product ions from sodiated beta-anilinodidrochalcones using an ion-trap mass spectrometer

The fragmentation reactions of sodiated beta-anilinodidrochalcones have been investigated by electrospray ionization multi-stage mass spectrometry (ESI-MS(n)). The fragment ion of sodiated N-benzylidenebenzenamine (P1) easily undergoes ion-molecule reactions with the residual ESI solvent molecules (H2O and CH3OH) in the vacuum system, as verified by MS3 and accurate MS analysis. The formed hydrated ions appear as an unusual leading peak in the profile spectrum, which results in a deviant decreasing mass shift of almost 1 Da. Density functional theory calculations indicate that P1 easily associates with H2O without any energy barrier. Thus, the hydrated P1 exists partially as a loose system of P1 and H2O, which provides a reasonable explanation for the decreasing mass shift of the solvated P1. The above results are important in obtaining structural information from MS(n) spectra and preventing erroneous data interpretation for the analogous adducts.