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The known antimicrobially active oxygenated terpenes, carvone, terpinen-4-ol, α-terpineol, carveol and myrtenol were found to cause significant K+ leakage from E. coli cells. In contrast, the relatively inactive oxygenated terpenes, 1,8-cineole and p-menth-6-ene-2,8-diol, did not cause significant K+ leakage. Slight structural differences between active terpenes were found to affect the rate of K+ leakage from E. coli cells. The membrane permeabilizing effect of the active terpenes is considered a significant factor in their antimicrobial activity.

Four pairs of oxygenated terpenes, with closely related chemical structures but considerably different minimum inhibitory concentration values (1) against P. aeruginosa or E. coli, showed differences in rate of cells killed over 2 h. Addition of polymyxin B nonapeptide (PMBN) as an outer membrane permeabilising agent was found to significantly increase the initial rates and overall numbers of cells killed for all compounds. The hydrocarbon limonene and the ester geranyl acetate, normally inactive, were also investigated. Both compounds had little killing effect when added alone to the cells but did show an enhanced killing capacity upon the addition of PBMN.

The presence of terpinen-4-ol in the steam distillates from Melaleuca argentea, M. dissitiflora and M. linariifolia can be linked to high levels of (+)-sabinene and cis- and trans-sabinene hydrate in the leaf material. Incorporation of 18O into the terpinen-4-ol from steam distillates performed in H2 18O confirms the generation of the terpinen-4-yl carbocation intermediate from (+)-sabinene and cis- and trans-sabinene hydrate. This same mechanism is also operating during leaf aging.

(Z)-β-ocimene has been found to occur in amounts of 85% in foliage of Homoranthus montanus Craven and S. R. Jones and 69–71% in H. flavescens Cunn. ex. Schauer. (E)-β-ocimene also occurred in amounts of 5–9% and 2–5% respectively. The oil yield in both cases was 0.3–0.5% based on fresh leaves.

Steam distillation of young leaves of Melaleuca alternifolia in 18O-labelled water resulted in the 180-labelling of the principal oxygenated p-menthanes, terpinen-4-ol and α-terpineol. The p-methanes; terpinen-4-ol, γ-terpinene, α-terpinene, terpinolene and p-cymene, found in M. alternifolia oil are primarily formed from cis-sabinene hydrate and to a lesser extent from trans-sabinene hydrate and sabinene. The pattern of the p-menthanes and the enantiomeric composition of terpinen-4-ol found in young and old leaves, regardless of the isolation process are similar to those formed during the acid catalyzed transformations of sabinene or sabinene hydrate. The incorporation of 180 into terpinen-4-ol during steam distillation in H2 18O supports the formation of terpinen-4-ol and the other p-menthanes from the terpinen-4-yl cation, generated from the hydration of sabinene and/or sabinene hydrate. This same mechanism appears to operate during leaf aging. Around 60% of the terpinen-4-ol obtained from the ...