Constituents, Enantiomeric Content, and ChE Inhibitory Activity of the Essential Oil from

The physical properties, chemical composition, enantiomer distribution, and cholinesterase (ChE) inhibitory activity were determined for a steam-distilled essential oil (EO), with a yield of 0.15 ± 0.05 % (w/w), from H. laricifolium aerial parts, collected in southern Ecuador. The oil qualitative and quantitative analyses were performed by GC-EIMS and GC-FID techniques, using two capillary columns containing a non-polar 5%-phenyl-methylpolysiloxane and a polar polyethylene glycol stationary phase, respectively. The main constituents (>10%) detected on the two columns were, respectively, limonene (24.29, 23.16%), (E)-β-ocimene (21.89, 27.15%), and (Z)-β-ocimene (12.88, 16.03%). The EO enantioselective analysis was carried out using a column based on 2,3-diethyl-6-tert-butyldimethylsilyl-β-cyclodextrin. Two mixtures of chiral monoterpenes were detected containing (1R,5R)-(+)-α-pinene (ee = 83.68%), and (S)-(-)-limonene (ee = 88.30%) as the major enantiomers. This finding led to some hypotheses about the existence in the plant of two enantioselective biosynthetic pathways. Finally, the EO exhibited selective inhibitory effects in vitro against butyrylcholinesterase (BuChE) (IC50 = 36.80 ± 2.40 µg/mL), which were about three times greater than against acetylcholinesterase (IC50 = 106.10 ± 20.20). Thus, the EO from Ecuadorian H. laricifolium is an interesting candidate for investigating the mechanism of the selective inhibition of BuChE and for discovering novel drugs to manage the progression of Alzheimer’s disease.