Isolation and characterisation of (-)-genifuranal: The principal antimicrobial component in traditional smoking applications of 'Eremophila longifolia' (Scrophulariaceae) by Australian aboriginal peoples

Abstract

Ethnopharmacological significance: 'Eremophila longifolia' is considered by some Australian Aboriginal tribal groups to be among the most significant of the medicinal plants in contemporary and traditional use. Usage modalities traditionally involved lipophilic extraction into animal fats and most importantly, ceremonial or medicinal smoking applications, involving the fumigation of mothers and infants following childbirth or boys following circumcision. An attempt was made to replicate the smoking modalities used by Australian Aboriginal people in the laboratory to identify bioactive compounds. Materials and methods: Two methods were used to produce smoke extracts; smoke was channelled through a condenser then bubbled into solvent, or bubbled directly into H₂O then partitioned into chloroform followed by butanol. Extracts were used, firstly for antimicrobial screening using micro-titre plate broth dilution to produce minimum inhibitory concentrations (MIC), and secondly for chemical analysis. Structure elucidation of an abundant compound isolated from the smoke extract was performed using 2D-NMR and derivatisation. Results: Significant antimicrobial activity (<1.0 mg/ml) was produced using the smoke extracts against the Gram-positive species 'Staphylococcus aureus', 'Bacillus subtilis' and the yeast 'Candida albicans'. A major component of the smoke with strong antimicrobial activity (0.13-0.5 mg/ml) was isolated which we have named (-)-genifuranal. Structure elucidation using 2D-NMR and derivatisation demonstrated genifuranal to be 5,6-dihydro-4H-cyclopenta[c]furan-4-ylacetaldehyde. Genifuranal is not observed in the leaves before heating, but is produced in the smoking or heating process and is thought to derive from hydrolysis and rearrangement of geniposidic acid or a related glycoside. Only geographically specific specimens of 'Eremophila longifolia' produced (-)-genifuranal, which strongly supports previous hypothesised geographical variation in traditional usage, reflective of phytochemical variation. Conclusion: It would appear that genifuranal is the medicinal principal involved in traditional use of 'Eremophila longifolia' when smoking modalities are used. Topical treatments traditionally produced by lipophilic extraction into animal fats are not likely to have had genifuranal present, as the mechanism for its formation requires heat.