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

Abstract

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.