Analysis of the major chiral compounds of Artemisia herba-alba essential oils (EOs) using reconstructed vibrational circular dichroism (VCD) spectra: En route to a VCD chiral signature of EOs

Abstract

An unprecedented methodology was developed to simultaneously assign the relative percentages of the major chiral compounds and their prevailing enantiomeric form in crude essential oils (EOs). In a first step the infrared (IR) and vibrational circular dichroism (VCD) spectra of the crude essential oils were recorded and in a second step they were modelized as a linear weighted combination of the IR and VCD spectra of the individual spectra of pure enantiomer of the major chiral compounds present in the EOs. The VCD spectra of enantiomer of known enantiomeric excess shall be recorded if they are not yet available in a library of VCD spectra. For IR, the spectra of pure enantiomer or racemic mixture can be used. The full spectra modelizations were performed using a well known and powerful mathematical model (least square estimation: LSE) which resulted in a weighting of each contributing compound. For VCD modelization, the absolute value of each weighting represented the percentage of the associate compound while the attached sign addressed the correctness of the enantiomeric form used to build the model. As an example, a model built with the non-prevailing enantiomer will show a negative sign of the weighting value. For IR spectra modelization, the absolute value of each weighting represented the percentage of the compounds without of course accounting for the chirality of the prevailing enantiomers. Comparison of the weighting values issuing from IR and VCD spectra modelizations is a valuable source of information: if they are identical, the EOs are composed of nearly pure enantiomers, if they are different the chiral compounds of the EOs are not in an optically pure form. The method was applied on four samples of essential oil of Artemisia herba-alba in which the three major compounds namely (-)-α-thujone, (+)-β-thujone and (-)-camphor were found in different proportions as determined by GC-MS and chiral HPLC using polarimetric detector. In order to validate the methodology, the modelization of the VCD spectra was performed on purpose using the individual VCD spectra of (-)-α-thujone, (+)-β-thujone and (+)-camphor instead of (-)-camphor. During this work, the absolute configurations of (-)-α-thujone and (+)-β-thujone were confirmed by comparison of experimental and calculated VCD spectra as being (1S,4R,5R) and (1S,4S,5R) respectively.

Chemical characterization and bioactivities assessment of Artemisia herba-alba Asso essential oil from South-western Algeria

“…Other components found in a small amount are camphene (3.6%), γ-terpinene (1.4%), borneol (1.3%), and p-cymene (1.0%). This chemotype is similar of those reported by Said et al (2016), with a percentage of 72.7%, 11.8%, 15.5% and 45.4%, 32.8%, 21.8% of α-thujone, β-thujone, and camphor respectively in Beni-Ouenif (Bechar, Algeria) and the Atlas Mountains (Merrakech, Morocco). However, αthujone as the major compound was also reported by Belhattab et al (2014) in Bougaa, Bousaada and Boutalleb, instead of camphor and chrysanthenone.…”

Chiroptical fingerprints to characterize lavender and lavandin essential oils

“…Indeed, the separation by GC for the enantiomers of the main markers of lavender EOs has already been published [35][36][37] and recently optimized by flow modulated comprehensive two-dimensional GC to be used for quality control [38] . On the other hand, chiroptical vibrational spectroscopy has been used by our group to analyze the enantiomeric composition of the markers in Artemisia herba-alba EOs [39] . In the case of lavender EOs, preliminary tests proved that the intensity of the signal for vibrational and electronic circular dichroism spectra was too weak to be used.…”

Artemisia arborescens Essential Oil Composition, Enantiomeric Distribution, and Antimicrobial Activity from Different Wild Populations from the Mediterranean Area

“…Pure (−)‐ α ‐thujone and (+)‐ β ‐thujone were also the main enantiomer in the oil of A . herba‐alba and Thuja occidentalis . In A .…”