A study of divalent metal cations Cu2+, Zn2+ and Pb2+ attachment to 3-hydroxyflavone, 5-hydroxyflavone and 5-methoxyflavone by nanoelectrospray ionization LTQ Orbitrap mass spectrometry

Cu2+, Zn2+ and Pb2+ cationized 3-hydroxyflavone, 5-hydroxyflavone and 5-methoxyflavone complexes were studied by nanoelectrospray ionization (nano-ESI) LTQ Orbitrap tandem mass spectrometry. With some exceptions, the most common types of ions observed in the nano-ESI mass spectra of Cu2+, Zn2+ and Pb2+ cationized 5-methoxyflavone are doubly and singly charged species [3L+M]2+, [2L+M]2+, [2L+M+NO3]+, [L+M+NO3]+ and [L−CH3+M]+ (where L = 5-methoxyflavone and M = Cu2+, Zn2+ and Pb2+). Nano-ESI spectra also show [L]+ and [L−H]+ types ions for Cu2+ and Pb2+ complexes. The losses of H2O, CHO and (CO + H2) are predominantly observed in the nano-ESI mass spectra of Cu2+ cationized 5-methoxyflavone complexes. Cu2+ is also unique in producing a variety of reduced Cu+ species. The common ions observed in the nano-ESI mass spectra of 3-Hydroxyflavone and 5-hydroxyflavone with divalent cations are [3L−3H+2M]+, [2L−H+M]+ and [L−H+M]+ where L = 3-hydroxyflavone or 5-hydroxyflavone. Some exceptions are also observed. For example [2L−3H+2Zn]+ is displayed by 3-hydroxyflavone but [2L−2H+2Cu]+ by 3-hydroxyflavone and 5-hydroxyflavone. The losses of CHO and (CHO + CO) are observed by Cu2+ and Zn2+ but not by Pb2+. Tandem mass spectra on [3L+M]2+, [2L+M]2+, [2L+M+NO3]+, [L+M+NO3]+ complexes for 5-methoxyflavone indicate that electronic configuration of the metal cations and counter ion NO3− has significant influence in the observed dissociation patterns. For example, CID of [2L+Cu]2+ and [2L+Pb]2+ for 5-methoxyflavone shows unique ions [L]+, [L−H]+, [L−H2O]+, [L−CHO]+ and [L−2H−CO2]+ that were not observed by Zn2+ complexes. Similarly the molecular ion [L]+ is displayed by the [L+NO3+Zn]+ complexes but not by [L+NO3+Cu]+ and [L+NO3+Pb]+. 3-Hydroxyflavone and 5-hydroxyflavone also show some interesting results. For example product ions that contain metal cations due to loss of CHO and CO are exclusively observed for the [L−H+Cu]+ complexes of 3-hydroxyflavone whereas reverse is observed for the [2L−H+Zn]+ complexes.

Graphical abstractFigure optionsDownload full-size imageDownload high-quality image (131 K)Download as PowerPoint slideHighlights► Nano-ESI spectra of all three flavones show losses of H2O, CHO and (CO + H2) induced by Cu2+ and Zn2+. ► Doubly charged complex ions of the form [3L+M]2+ and [2L+M]2+ are observed for 5-methoxyflavone. ► CID of [2L+Cu]2+ and [2L+Pb]2+ for 5-methoxyflavone show extensive ligand fragmentation. ► Fragment ions due to loss of CHO and CO are seen in the CID of the [L−H+Cu]+ ion, L = 3-hydroxyflavone.