Structural characterization of the ligstroside aglycone isoforms in virgin olive oils by liquid chromatography–high‐resolution Fourier‐transform mass spectrometry and H/Dexchange

Abstract

A systematic structural characterization of the isomeric forms related to ligstroside aglycone (LA), one of the most relevant secoiridoids contained in virgin olive oils, was performed using reverse phase liquid chromatography with electrospray ionization Fourier‐transform single and tandem mass spectrometry, operated in negative ion mode (RPLC‐ESI(−)‐FTMS and FTMS/MS). The high mass resolution and accuracy provided by the adopted orbital trap mass analyzer enabled the recognition of more than 10 different isomeric forms of LA in virgin olive oil extracts. They were related to four different types of molecular structure, two of which including a dihydropyranic ring bearing one or two aldehydic groups, whereas the others corresponded to dialdehydic open‐structure forms, differing just for the position of a C═C bond. The contemporary presence of enolic or dienolic tautomers associated to most of these compounds, stable at room temperature (23°C), was also assessed through RPLC‐ESI‐FTMS analyses operated under H/D exchange conditions, ie, by using D2O instead of H2O as co‐solvent of acetonitrile in the RPLC mobile phase. As discussed in the paper, the results obtained for LA indicated a remarkable structural similarity with oleuropein aglycone (OA), the most abundant secoiridoid of olive oil, whose isoforms had been previously characterized using the same analytical approach.

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

“…The latter were able to explain the generation of several other product ions (detected especially in MS/MS spectra related to peaks #1, since they were the most abundant ones for both compounds) through neutral losses described in the scheme. Not surprisingly, fragmentation pathways leading to those product ions were generally similar to those previously observed for the aglycones of oleuropein and ligstroside [17,18].…”

“…The key step for the generation of secoiridoid isoforms with a C 8 =C 10 bond is the subsequent re-opening of cyclic structures generated by 1,4-intramolecular Michael addition. Indeed, this process can either return open structures with the original C 9 =C 10 bond or new open structures in which the C=C bond is located between C 8 and C 10 [17,18]. These reactions likely occur in the first stages of the olive oil production (especially during drupes crushing and malaxation) and might be modulated by the specific conditions adopted during those stages, thus explaining why the incidence of secoiridoid isoforms with a C 8 =C 10 bond can change remarkably between different olive oils.…”

“…Separations were performed using the following elution gradient, based on water (solvent A) and acetonitrile (solvent B), already adopted in our laboratory for the separation of isomeric secoiridoids in olive oil extracts [17,18]: 0-5 min) 20% solvent B; 5-35 min) from 20 to 50% (v/v) solvent B; 35-40 min) from 50 to 100% of solvent B; 40-50 min) isocratic at 100% solvent B; 50-55 min) from 100 to 20% of solvent B; 55-70 min) column reconditioning at 20% solvent B. The flow rate was always set at 200 µL/min and the column temperature at 25 • C. Olive leaves/drupes secoiridoids were detected as [M-H] − ions, generated upon deprotonation occurring during the ESI process.…”

Bioactive Compounds in Waste By-Products from Olive Oil Production: Applications and Structural Characterization by Mass Spectrometry Techniques

“…More recently, a detailed characterization of the complex set of isomeric forms found for major secoiridoids present in olive oil has been reported by Abbattista et al using an hybrid quadrupole-Orbitrap mass spectrometer, capable of high collisional energy MS/MS analyses under high resolution/accuracy conditions, and implementing H/D exchange experiments to recognize stable enolic tautomers related to aglyconic secoiridoids [110,111]. The described approach appears very promising for the search for isomeric secoiridoids also in olive leaves extracts.…”

Influence of organo-mineral supplementation on the production of secondary metabolites in in vitro-germinated Bromelia balansae Mez.

“…Although quantitative analysis showcased higher concentration of phenols and flavonoids in the leaves and roots from organo-mineral-supplemented plants, these spectrophotometric tests promote the overlapped detection of both the glycoside as well as the aglycone of secondary metabolites (Abbattista et al, 2019;Akimoto et al, 2017). Considering that the participation of secondary metabolites in biochemical processes in plants occurs mostly as glycosylated compounds (Jamwal et al, 2018), it can be suggested that the lower relative abundances of the aglycone signals in the supplemented leaves and roots therefore hints that these compounds predominate as glycosides.…”