Characterization of phenolic compounds using separation techniques coupled to mass spectrometry of plant extracts with demonstrated bioactivity

Abstract

This doctoral thesis, entitled "Characterization of phenolic compounds using separation techniques coupled to mass spectrometry of plant extracts with demonstrated bioactivity", consists of two main sections: the Introduction and the Experimental Section.

The first section describes an overview of traditional medicine and functional food, plant foods as natural source of bioactive compounds, polyphenols and bioactivity and the analytical tools used to identify polyphenols such as high-performance liquid chromatography (HPLC) and ultraperformance liquid chromatography (UPLC) coupled to different detectors such as UV-visible (UV-vis) and mass spectrometry (MS) with ion-trap (IT), time-of-flight (TOF), and quadrupole time-of-flight (Q-TOF) analyzers.

The experimental section is divided in several chapters as follows:

Chapter 1 provides an overview of the uses of different powerful analytical tools such as HPLC and capillary electrophoresis (CE) coupled to different detection systems for the characterization of anthocyanins in fruits and vegetables, discussing the fundamentals of sample extraction and separation.

Chapter 2 shows the characterization of phenolic and other polar compounds in whole zucchini (Cucurbita pepo L.) fruit. An extraction method has been applied in order to extract the polar fraction from the samples studied, followed by HPLC separation assisted by diode array detector (DAD) and Q-TOF-MS. To the best of our knowledge, 10 phenolic acids, 16 flavonoids, and 17 other polar compounds with their derivatives are reported here in the whole zucchini fruit for the first time.

Chapter 3 demonstrates a powerful analytical method to separate and characterize phenolic compounds in aqueous and ethanolic rooibos (Aspalathus linearis) extracts, as rooibos is a rich source of polyphenols. This plant is used to make a mild-tasting caffeine-free tea that is low in tannins compared to green or black teas and has antioxidant and antimutagenic/antitumoral properties. The method, combining HPLC-ESI-TOF-MS and HPLC-ESI-IT-MS2 experiments confirmed the presence of 25 and 30 phenolic compounds in the aqueous and ethanolic extracts, of which 11 are reported here for the first time in rooibos. In addition, the rooibos tea extract showed its highest bioactivity in the groups fed with the high-fat high-cholesterol diet (HF), lowering serum cholesterol and triglyceride concentrations compared to control.

Finally, the last chapter is divided into two subchapters. The first subchapter 4A concerns the technique of HPLC-DAD-TOF-MS, using negative and positive ionization modes, to characterize commercial cranberry syrup used to prevent urinary-tract diseases. It was thus possible to identify 34 different compounds, 9 of which have been tentatively characterized for the first time in cranberry syrup. With regard to antibacterial activity, it was possible to demonstrate that very low concentrations of cranberry extract have can modify the non-specific adherence properties of E. coli, reducing surface hydrophobicity. Following the approach of this subchapter, subchapter 4B shows the powerful use of UPLC coupled to Q-TOF MS, for identifying polyphenols and their metabolites in human urine after a single dose of cranberry syrup. 9 native phenolics together with 23 conjugated metabolites, including isomers, were identified in urine using this optimized method.