Stereoisomeric interactions of α- and β-pinene with essential oil compounds

This study investigated the stereoselective antimicrobial, anti-quorum sensing, and toxicity profiles of the enantiomers of α- and β-pinene, individually and in 1:1 combinations with 14 essential oil compounds. Minimum inhibitory concentration (MIC) assays revealed comparable activity between the α- and β-pinene enantiomers (0.25-4.00 mg/mL) when tested independently, but notable enantioselective differences in combination. S-(-)-α-pinene showed greater synergistic interactions with β-caryophyllene, estragole, and ocimene against Staphylococcus aureus (ΣFIC = 0.50), whereas R-(+)-β-pinene was synergistic with isoeugenol against Cryptococcus neoformans (ΣFIC = 0.50). Anti-quorum sensing assays using Chromobacterium violaceum demonstrated stronger inhibition by R-(+)-α-pinene (MQSIC = 0.38 ± 0.18 mg/mL; 67.02 % violacein inhibition at 0.50 mg/mL) and R-(+)-β-pinene (MQSIC = 0.50 mg/mL; 57.32 %) compared with their S-(-) counterparts (> 0.50 mg/mL; 30.72 % and 22.78 %, respectively). The most pronounced stereochemical variation was observed in combinations with γ-terpinene and ocimene (> 20 % difference in violacein inhibition). Brine shrimp lethality assays showed both α-pinene enantiomers to be non-toxic, while S-(-)-β-pinene exhibited greater toxicity (90.82 % mortality) than R-(+)-β-pinene (30.75 %) after 48 h. These findings highlight the important role of stereochemistry in determining the bio-activity and safety of α- and β-pinene, highlighting the therapeutic importance of chirality in the research of essential oil compounds.