Ion mobility mass spectrometry with molecular modeling to reveal bioactive isomer conformations and underlying relationship to isomerization

In the medicine and drug development, molecular modelling is an important tool. It is attractive to develop a platform connecting the theoretical structure modelling and the results from experimental measurement. In addition, separation and structure analysis of bioactive constituent isomers are still challenging. Drift tube ion mobility (IM)-mass spectrometry (MS) provides the experimental collision cross section (CCS) which contains the structural information. The experimental CCS can be compared with the calculated CCS of the molecular modelling structures. This technique is especially useful for bioactive constituents in herbal medicine because active isomers with the same chemical formula are common in these samples. IM helps separate, identify these isomers and reveal the details about their structures and conformations. Two model bioactive constituents, caffeoylquinic acids (CQAs) and dicaffeoylquinic acids (di-CQAs), were selected to systematically investigate the influence of solution, ion source conditions and ion heating to the isomer CCS distributions. By comparing the calculated CCS to the experimental value, we identified the favorable conformations of CQA. The most compact conformation of the CQA was less likely to isomerize compared to the more extended conformations. It was found that the isomerization tendency agreed with the conformation favorability. This study offers an effective approach to predict and demystify the herbal medicine active constituents' conformation and isomerization.