Gas-phase ligation of Fe+ and Cu+ ions with some flavonoids

It was assumed that gas-phase ligation of metal monocations by flavonoids might provide some insight on the intrinsic antioxidant activity of the latter. Thus, the ligation of Fe+ and Cu+ ions by apigenin ( 1), luteolin ( 2), kaempferol ( 3), quercetin ( 4), myricetin ( 5), and naringenin ( 6) was investigated in the gas phase in a Fourier transform mass spectrometer ( FTMS). Both of the metal ions, which were produced by laser desorption ionization ( LDI), bind consecutively to two neutral flavonoid molecules either with or without the simultaneous loss of some part ( H, CO, H2O) of the latter. The flavonoids are present in the instrument at steady concentrations. The formation of flavonoid positive ions by charge exchange is also a common observation but is accompanied, in some cases, by a loss of H, CO, or H2O fragments. The reaction paths and observed fragmentations are presented. The results are supported by DFT B3LYP calculations that indicate a preference for metal ion attack at C-ring and not at the B-ring site considered to be mainly responsible for flavonoid antioxidant activity.