Chemical composition and antimicrobial activity of the essential oils of Onychopetalum amazonicum R.E.Fr.

Abstract The essential oils from leaves, twigs and trunk bark of Onychopetalum amazonicum R.E. Fr. (Annonaceae), obtained by hydrodistillation, were analysed by GC and GC–MS, and also were evaluated for in vitro antimicrobial activity. Forty-one compounds, which correspond to 75.0–92.2% of the oil components, were identified. Major compounds were sesquiterpenes, including (E)-caryophyllene, caryophyllene oxide, spathulenol, α-gurjunene, allo-aromadendrene and α-epi-cadinol. The oils were evaluated for antimicrobial activities against four bacteria strains and five pathogenic fungi. The oil of the trunk bark exhibited good activity against Staphylococcus epidermidis ATCC 12228, Escherichia coli ATCC 10538 and Kocuria rhizophila ATCC 9341, with a minimal inhibitory concentration of 62.5 μg/mL. The essential oil composition and the antimicrobial evaluation are reported for the first time for the genus Onychopetalum.


Introduction
Annonaceae is a pantropical family whose essential oils are predominantly constituted by monoterpenes and sesquiterpenes (Fournier et al. 1999). Recent investigations on essential oils obtained from Annonaceae revealed interesting biological properties, such as antimicrobial and antitumor activities (Ferraz et al. 2013). Although extensively explored from the phytochemical viewpoint, some genera remain unexplored in Annonaceae. For instance, Onychopetalum presents this lack; only one work describing its alkaloidal content was previously reported (Almeida et al. 1976).
As a continuation of previous studies on the chemical composition and antimicrobial evaluation of species belonging to unexplored genera in Annonaceae (Soares et al. 2015), we described herein the chemical composition and antimicrobial activity of the essential oils from aerial parts of O. amazonicum.

Essential oil analysis
The relative yield (% w/w) of the three essential oils from the O. amazonicum was 0.18% for leaves, 0.34% for twigs and 0.37% for trunk bark; all oils presented a yellow colour. Chemical analysis by GC and GC-MS showed a total of 41 compounds, which represent between 75.0 and 92.2% of all components of the oils (Table S1). Sesquiterpene hydrocarbons (27.5-60.7%) were observed as the main group for the leaves (60.7%) and trunk bark (56.9%) oils, whereas oxygenated sesquiterpenes (27.5-47.5%) were observed as the major class for the twigs oil (47.5%).
Previous studies describe the compounds reported in this work in several genera of the Annonaceae family, e.g. Annona (Thang et al. 2013), Dasymaschalon  and Fissistigma . Although no data regarding the essential oils composition of genus Onychopetalum were previously reported, studies on close-related genera Unonopsis and Bocageopsis demonstrate that sesquiterpenes are also a majority in the essential oils composition Soares et al. 2015). This observation is in agreement with previous studies performed with Annonaceae species, for which sesquiterpenes are mainly found in leaves, bark and roots oils, whereas monoterpenes are commonly observed in fruits and seeds (Fournier et al. 1999).

Antimicrobial activity
Only the essential oil of the trunk bark of O. amazonicum presented interesting activity (MIC 62.5 μg/mL) (Ríos & Recio 2005) against gram-positive bacteria Kocuria rhizophila ATCC 9341 and Staphylococcus epidermidis ATTC 12228, and gram-negative bacteria Escherichia coli ATCC 10538, which was not observed for the other oils (Table S2). This good activity observed (Holetz et al. 2002) may be associated with the presence of the secondary compound allo-aromadendrene (21.2%), once α-epi-cadinol (24.1%) and α-gurjunene (14.9%) were the main compounds in the non-active twigs essential oil. In a recent study performed with essential oil of the trunk bark from the close-related genus Bocageopsis, activity against S. epidermidis (MIC 250 μg/mL) was observed and associated with the presence of the secondary compounds (Soares et al. 2015). Previous biological studies conducted with samples from Monanthotaxis discolor (Annonaceae), which are rich in allo-aromadendrene (11.7-12.8%), demonstrated mild antimicrobial activity (Parmena et al. 2012).

Conclusion
The present study described the chemical composition of essential oils of O. amazonicum (Annonaceae), for which 41 compounds were identified. The presence of sesquiterpenes and absence of monoterpenes are in agreement with the chemistry of the Annonaceae family. The good antimicrobial activity observed in the trunk bark essential oil against K. rhizophila ATCC 9341, S. epidermidis ATTC 12228 and E. coli ATCC 10538 bacteria improves the biological knowledge about the essential oils of the genus Onychopetalum. The results presented here are the first reports about the chemical composition and the antimicrobial activities of the essential oils from Onychopetalum.

Supplementary material
Experimental details related to this article are available online, alongside Tables S1 and S2.