Preparation of quercetin glucuronides and characterization by HPLC-DAD-ESI/MS

Quercetin is the main flavonol in the human diet, and the most commonly used in studies of biological activity. The major circulating forms of quercetin found in the human plasma after consumption of food containing distinct quercetin glycosides are glucuronides and sulfates. In this work quercetin glucuronides have been obtained from green beans (quercetin 3-glucuronide) and by enzymic synthesis (quercetin 4'-glucuronide) using a modification of the method described by Plumb et al. (Methods in polyphenol analysis, The Royal Society of Chemistry, Cambridge, pp 187, 2003) so as to improve the original low yields of that methodology. The method finally optimised got yields of 19% in the preparation of quercetin 4'-glucuronide, which allows its further isolation for their use in biological assays. In addition, quercetin 3'-glucuronide, 3-glucuronide and a diglucuronide were synthesised with lower yields. The compounds prepared have been employed to perform assays in order to obtain data for their identification by HPLC coupled to photodiode array detection and tandem mass spectrometry. It was observed that the analysis by HPLC-ESI/MS/MS could allow the identification of different quercetin glucuronides based on the presence of some minor key MS2 fragments.