Fragmentation characteristics and isomeric differentiation of flavonol O ‐rhamnosides using negative ion electrospray ionization tandem mass spectrometry

RATIONALE Flavonoids exhibit a wide range of biological activities. The differentiation and structural characterization of the flavonol O ‐rhamnosides and their isomers are important research topics. Electrospray ionization tandem mass spectrometry (ESI‐MS n ) is an indispensable tool and has become an efficient method for the identification of bioactive natural products. METHODS Negative ion ESI‐MS n was used to characterize and differentiate flavonol O ‐rhamnosides and their isomeric compounds. The collision energies in collision‐induced dissociation were varied to optimize the structural information. RESULTS Flavonol 7‐ O ‐rhamnosides containing either a hydroxyl or a methoxyl group at the C‐8 position clearly differed from their isomeric compounds, such as the flavonol 3‐ O ‐rhamnosides, in the product ion spectra of the [M–H] – ions. The differences in the locations of the glycosyl group and OCH 3 group resulted in a significant change in the relative abundances of the Y 0 – and [Y 0 –H] –• ions, and the formation of some characteristic ions, such as 1,3 A 0 – and [ 1,3 A 0 –CH 3 ] –• . CONCLUSIONS An efficient ESI‐MS n method for the structural identification and isomeric differentiation of flavonol O ‐rhamnosides was developed. The differences in the glycosyl type, and the glycosylation and methoxylation positions in the structures of isomeric flavonoids, can be determined using (–)ESI‐MS n spectra. The formation and relative abundances of [Y 0 –H] –• and Y 0 – ions, as well as the comparison of some diagnostic ions, are vital in the differentiation of isomeric flavonoids. These results have practical applications in the rapid identification of flavonol O ‐rhamnosides present in crude bioactive extracts. Copyright © 2012 John Wiley & Sons, Ltd.