Tandem mass spectrometric analysis and density functional theory calculations on the fragmentation behavior of two tetradecanoylingenol regioisomers from Euphorbia wallichii

RATIONALE Two structurally similar bioactive regioisomers, 3‐O‐tetradecanoylingenol and 20‐O‐tetradecanoylingenol, from Euphorbia wallichii presented quite different fragmentation behaviors. Revealing the related fragmentation pathways may improve the efficiency of characterization and identification of such type of compounds. METHODS The two regioisomers were subjected to collision‐induced dissociation (CID) on Finnigan LCQDECA and LTQ Orbitrap XL instruments. Based on the CID results, the possible fragmentation pathways were proposed and investigated with density functional theory (DFT) calculations. RESULTS Elimination of C14H28O2 (tetradecanoic acid) for 3‐O‐tetradecanoylingenol and the sequential losses of C4H8 (butylene) for 20‐O‐tetradecanoylingenol were observed in ESI‐MS/MS, witnessed also by HR‐ESI‐MS/MS. The fragmentation pathways were proposed and verified by calculating the activation energy of their transition states by DFT calculations. CONCLUSIONS Based on the observations, fragmentation pathways for the two regioisomers were proposed and verified by calculating the energy of the reactants, products and the corresponding transition states using DFT. This report should have value in rapid identification of the derivatives of ingenol and other regioisomers in natural products. Copyright © 2012 John Wiley & Sons, Ltd.