Investigating the thermal stability of six caffeoylquinic acids employing rapid-resolution liquid chromatography with quadrupole time-of-flight tandem mass spectrometry

Abstract

The caffeoylquinic acids (CQAs) are one important group with various biological activities of natural products in herbal medicines and food plants. This finding caused new interest in the properties and thermal stability of CQAs, since many processing and preparation of pharmaceutical and foodstuffs involved liquid extraction and heating procedure. A rapid-resolution liquid chromatography method coupled with electrospray ionization quadrupole time-of-flight tandem mass spectrometry has been developed to research the thermal stability and transformation products of the six CQAs, 5-O-CQA, 3-O-CQA, 4-O-CQA, 3, 4-O-di-CQA, 4, 5-O-di-CQA and 3, 5-O-di-CQA, in water by heating an aqueous solution of each compound in the boiling water for 0–6 h. The order of thermal stability is 5-O-CQA > 3-O-CQA > 4-O-CQA for mono-CQAs. However, it is different with the research results reported by Kuhnert et al.: When the mono-CQAs were treated with aqueous tetramethylammonium hydroxide (pH 12) at the room temperature, the order of the stability is 5-O-CQA > 4-O-CQA > 3-O-CQA in terms of the hydrolysis of the caffeoyl ester. For di-CQAs, the order of thermal stability is 4, 5-O-di-CQA > 3, 5-O-di-CQA > 3, 4-O-di-CQA during heated in the boiling water bath. The mono-CQAs undergo transformations such as isomerization, hydroxylation and/or transformation to caffeic acid and quinic acid. The di-CQAs could isomerize to each other and degrade to mono-CQAs, caffeic acid and the compounds with the formula of C15H14O6. It is useful for researchers to choose the proper sample procedure (extraction method and time) and investigate plant metabolism.