Complexation of Divalent Metals by Natural Furoquinoline Alkaloids: An Electrospray Mass Spectrometry and DFT Study

Rationale Despite the well‐established metal‐chelating properties of 8‐hydroxyquinoline and related compounds, little is known about the interaction of furoquinoline alkaloids with biologically relevant metal ions. This study highlights the value of combining ESI‐MS, energy‐resolved mass spectrometry breakdown experiments, and DFT approaches to characterize metal‐binding properties of methoxy‐substituted furoquinoline alkaloids, enabling the differentiation of positional isomers through characteristic metal‐adduct formation and ion fragmentation. Methods The complexation behavior of the studied alkaloids toward Mg 2+ , Ca 2+ , Co 2+ , Cu 2+ , and Zn 2+ was studied by electrospray ionization mass spectrometry in a micrOTOF Q II mass spectrometer. Energy‐resolved mass spectrometry breakdown experiments were performed to assess the relative stability of the observed adducts. Adduct profiles were graphically compared by PCA of the ESI/MS dataset. Binding modes were evaluated by DFT calculations by B3LYP 6‐311+G(2d,3p). Results Quinolines 1 ‐ 5 yielded stable metal adducts with distinct profiles. Ligand oxidation to [M] +. and Cu(I) adduct formation occurred mainly for furoquinolines 1‐3 . Methyl‐loss fragmentation characterized compounds with an 8‐methoxyquinoline motif. Isomeric furoquinolines showed markedly different metal‐binding adduct profiles. Flindersiamine 1 showed the strongest metal‐binding affinity, yielding intense [3M+Me] 2+ and [2M+Me] 2+ adducts, and DFT calculations support its ability to act as an N,O‐bidentate ligand. Conclusions Metal‐adduct profiles obtained by ESI‐MS, supported by PCA analysis, provide a robust analytical framework for differentiating positional isomers of methoxy‐substituted furoquinoline alkaloids. ESI‐MS and DFT studies identified the 8‐methoxy substituent as a key determinant of metal complexation in furoquinoline alkaloids, revealing flindersiamine 1 as the strongest Cu 2+ ‐binding ligand and providing a basis for future studies on the biological properties of these compounds.