Exploring the Isomeric Precursors of Olive Oil Major Secoiridoids: An Insight into Olive Leaves and Drupes by Liquid-Chromatography and Fourier-Transform Tandem Mass Spectrometry.

Ramona Abbattista,Tommaso R I Cataldi,Ilario Losito,Cosima Damiana Calvano

doi:10.3390/foods10092050

Abstract

Secoiridoids play a key role in determining health benefits related to a regular consumption of extra-virgin olive oil (EVOO), in which they are generated from precursors of the same class naturally occurring in drupes and leaves of the olive (Olea europaea L.) plant. Here, reversed-phase liquid chromatography coupled to electrospray ionization and Fourier-transform single/tandem mass spectrometry (RPLC-ESI-FTMS and MS/MS) was employed for a structural elucidation of those precursors. The presence of three isoforms in both matrices was assessed for oleuropein ([M-H]− ion with m/z 539.1770) and was emphasized, for the first time, also for ligstroside (m/z 523.1821) and for the demethylated counterparts of the two compounds (m/z 525.1614 and 509.1665, respectively). However, only the prevailing isoform included an exocyclic double bond between carbon atoms C8 and C9, typical of oleuropein and ligstroside; the remaining, less abundant, isoforms included a C=C bond between C8 and C10. The same structural difference was also observed between secoiridoids named elenolic acid glucoside and secoxyloganin (m/z 403.1246). This study strengthens the hypothesis that secoiridoids including a C8=C10 bond, recently recognized as relevant species in EVOO extracts, arise mainly from specific enzymatic/chemical transformations occurring on major oleuropein/ligstroside-like precursors during EVOO production, rather than from precursors having that structural feature.

Highlights

Extra-Virgin Olive Oil (EVOO) has been an essential component of Mediterranean diet for centuries
Some differences in the profiles of product ions obtained for oleuropein and oleuroside were evidenced by Quirantes-Piné et al [27] after MS/MS analysis, yet an interpretation for this finding was not reported. Starting from this background, the main goal of this work is a systematic structural investigation, based on RPLC-ESI-FTMS and MS/MS, on the isoforms related to oleuropein and ligstroside, to their demethylated forms and to elenolic acid glucoside and secoxyloganin in extracts of drupes and leaves collected from trees of a typical Italian olive cultivar, Coratina, whose oil is known to be rich in secoiridoids [33]
The coupling between RPLC and high-resolution single and tandem mass spectrometry enabled a new, intriguing insight on major secoiridoids occurring in olive leaves and drupes, which represent the precursors of compounds of the same class acting as bioactive components in olive oil

Summary

Introduction

Extra-Virgin Olive Oil (EVOO) has been an essential component of Mediterranean diet for centuries. Phenolic compounds have been reported to be significantly implicated in EVOO health benefits, with species belonging to the class of secoiridoids playing a major role among them [2,3,4,5,6,7,8,9]. Secoiridoids are key secondary metabolites of Oleaceae plants, including Olea europaea L., whose drupes are employed in olive oil production. Major compounds of this class in olive drupes and leaves, generally known as oleuropein and ligstroside (see Figure 1), arise from the formation of an ester linkage between phenolic alcohols known as 3-hydroxy-tyrosol (3,4dihydroxy-phenylethylethanol, 3,4-DHPEA) and tyrosol (3-hydroxy- phenylethylethanol, 3-HPEA) and elenolic acid glucoside ( known as oleoside 11-methyl ester) [10,11].

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