Essentials Oils from Brazilian Eugenia and Syzygium Species and Their Biological Activities

The Eugenia and Syzygium genera include approximately 1000 and 1800 species, respectively, and both belong to the Myrtaceae. Their species present economic and medicinal importance and pharmacological properties. Due to their chemical diversity and biological activity, we are reporting the essential oils of 48 species of these two genera, which grow in South America and found mainly in Brazil. Chemically, a total of 127 oil samples have been described and displayed a higher intraspecific and interspecific diversity for both Eugenia spp. and Syzygium spp., according to the site of collection or seasonality. The main volatile compounds were sesquiterpene hydrocarbons and oxygenated sesquiterpenes, mainly with caryophyllane and germacrane skeletons and monoterpenes of mostly the pinane type. The oils presented many biological activities, especially antimicrobial (antifungal and antibacterial), anticholinesterase, anticancer (breast, gastric, melanoma, prostate), antiprotozoal (Leishmania spp.), antioxidant, acaricidal, antinociceptive and anti-inflammatory. These studies can contribute to the rational and economic exploration of Eugenia and Syzygium species once they have been identified as potent natural and alternative sources to the production of new herbal medicines.


Introduction
From the latter 19th century through the second half of the 20th century, Syzygium species were included in the Eugenia genus. However, anatomical and morphological analyses provided evidence that these genera were not closely linked, and a distinction between individuals belonging to these two genera has been attributed [1]. Eugenia and Syzygium genera belong to the Myrtaceae family, included in the Myrtales order, Rosidae clade, and Malvidae subclade [2]. In recent studies, these genera have been classified into subgenera; Eugenia includes Eugenia, Hexachlamys, and Pseudeugenia subgenera [3], while the Syzygium genus encompasses Syzygium, Acmena, Sequestratum, Perikion, Anetholea, and Wesa subgenera [4].

Bibliographic Search Criteria
The bibliographic research was performed using the databases Google Scholar, Pubmed, Science Direct, Medline, and Scopus. The keywords applied were "Eugenia", "Syzygium" and "essential oils"; and "volatile compounds", and "essential oils". Some unusual or incorrect botanical names were updated based on The Plant List" (http://www.theplantlist.org/).

Volatile Profiles
Eugenia essential oils are characterized by intra-and interspecific chemical diversity, with the predominance of cyclic sesquiterpenes, follow by monoterpenes, and without phenylpropanoids as the main compound (See Appendices A and B) [22]. However, dimethylxanthoxylin (73.2 and 83.0%), an acetophenone derivative, was identified by GC-MS and NMR techniques as the major constituent in EOs of two specimens of E. protenta McVaugh collected in Santarém Novo, western Pará state located in the Brazilian Amazon [23].

Seasonal Variation of Oil Composition
Some studies have shown variation in the chemical composition and yield of EOs of Eugenia and Syzygium affected by seasonality. Samples of fresh leaves of E. astringens (syn. Eugenia rotundifolia), with Brazil's occurrence, were collected every three months. The oil was mainly composed of cyclic monoterpene and sesquiterpene hydrocarbons, as α-pinene (19.7-34.4%), β-pinene (20.6-34.1%), and (E)-caryophyllene (3.6-11.7%). The rain precipitation data displayed a significant positive and negative correlation between the (E)-caryophyllene and α-pinene content, respectively [26].

Seasonal Variation of Oil Composition
Some studies have shown variation in the chemical composition and yield of EOs of Eugenia and Syzygium affected by seasonality. Samples of fresh leaves of E. astringens (syn. Eugenia rotundifolia), with Brazil's occurrence, were collected every three months. The oil was mainly composed of cyclic monoterpene and sesquiterpene hydrocarbons, as α-pinene (19.7-34.4%), β-pinene (20.6-34.1%), and (E)-caryophyllene (3.6-11.7%). The rain precipitation data displayed a significant positive and negative correlation between the (E)-caryophyllene and α-pinene content, respectively [26].
The leaf oils of Syzygium jambos (L.) Alston, collected in different year seasons, showed an environmental influence due to the foliar nutrients (N, Mn, Co, Fe, S, and Mg), and soil nutrients (Na, Al, S and H +, and Al).

Differences in Oil Composition and Extractions Methods
Differences in EO's yields and chemical composition may be associated with the extraction technique employed and its conditions. Some studies compared the extraction methods and reported their influence in yields and chemical compositions of oils [74]. The leaf oil of E. involucrata collected in Rio de Janeiro (Brazil) was rich in β-elemene (41.8%), bicyclogermacrene (28.4%) and (E)-caryophyllene (6.7%) when extracted by CO 2 supercritical fluid. However, the oil showed quantitative variations of β-elemene (42.4%), bicyclogermacrene (23.0%), and (E)-caryophyllene (13.4%) when extracted by hydrodistillation [37].

Antibacterial and Antifungal Activity
In the last years, antimicrobial resistance to antibiotics has been increasing due to the adaptive evolution of bacteria and fungi. For this reason, research focused on the potential of new antimicrobial agents based on natural products has been explored, particularly the essential oils [75].

Acetylcholinesterase Inhibition
Acetylcholinesterase (AChE) acts in the final step of the nervous impulse transmission by hydrolysis of acetylcholine (ACh), a neurotransmitter. AChE inhibitors reduce the ACh level in the brain and increase its concentration in the synapses [79]. In this sense, these inhibitors represent major compounds approved for clinical use in the symptomatic management of Alzheimer's disease and other neurodegenerative disorders [80]. In recent years, natural compounds from essential oils have shown a high anticholinesterase potential [79].

Cytotoxic Activity
The anticancer potential of essential oils has been widely studied, aiming to mitigate the resistance development to multiple drugs and side effects associated with antitumor drugs currently used [81]. Essential oils of E. egensis, E. flavescens, E. patrisii, E. polystachya and E. uniflora showed cytotoxic activity against human cancer cell lines. The main human carcinoma cell lines evaluated were colorectal (HCT-116), gastric (AGP-01), and melanoma (SKMEL-19) [37,48].

Antiprotozoal Activity
The protozoal or parasitic diseases induce significant morbidity and mortality, being endemic to developing countries. Thus, affordable drugs have serious side effects, high cost, and low effectiveness. Moreover, there is an increased need to expand the investigations for new drug development [82]. In this way, essential oils could be promising sources of antiprotozoal agents, opening perspectives to discover more effective drugs of vegetal origin, in the treatment of diseases caused by protozoa [83].
The essential oils of E. uniflora and S. cumini showed anti-Leishmanial activity. Eugenia uniflora oil from Brazil, rich in curzerene (47.3%), displayed significant anti-Leishmanial activity against promastigote (IC 50 , 3.04 µg/mL) and amastigote (IC 50 1.92 µg/mL) forms of Leishmania amazonensis. The oil was 20 times more toxic to amastigotes than to healthy macrophages. Although the anti-Leishmanial activity was not mediated by nitric oxide production, the authors have suggested that macrophage activation may be involved in the anti-Leishmanial activity of Eugenia uniflora essential oil, as evidenced by increases in both the phagocytic capacity and lysosomal activity [47]. Moreover, the essential oil of Syzygium cumini collected in Maranhão state (Brazil), rich in α-pinene (31.85%), showed anti-Leishmanial activity against promastigote form of L. amazonensis (IC 50 60 µg/mL) [67].

Antioxidant Aactivity
The antioxidants usually act as free radical scavengers, preventing oxidative stress, decreasing the possibility of chronic and degenerative diseases, controlling the autoxidation, and interrupting the propagation of free radicals, or by inhibiting the formation of free radicals via different mechanisms. Some aromatic plants are sources of natural antioxidants [83][84][85][86]. The antioxidant activity of Eugenia essential oils has been evaluated by radical scavenging DPPH, ABTS, and TLC methods, lipid peroxidation by β-carotene/linoleic acid, and the iron ion reduction power.

Conclusions
The Eugenia and Syzygium species from South America have high chemical variability and several biological activities in their essential oils. Based on the present study, these variations can be

Conclusions
The Eugenia and Syzygium species from South America have high chemical variability and several biological activities in their essential oils. Based on the present study, these variations can be attributed to different geographic occurrences or seasonal changes. Despite the medicinal use of Eugenia species in traditional therapy, few reports have been made about the biological activity of its essential oils (less than 5% of species in Brazil). On the other hand, the composition of oils from a large number of species (127 samples) is already known, representing 12.2% of the total species distributed in Brazil. The variations in the chemical profile of these species indicated the importance of optimizing the protocols for collecting, processing, and extracting plant material. The standardization of these essential oils can contribute to the economic and commercial exploitation of bioactive products from aromatic plants.

Acknowledgments:
The authors are grateful to CNPq and Capes, institutions for scientific support and scholarship of the Brazilian Government.

Appendix B
Biomolecules 2020, 10, x FOR PEER REVIEW 26 of 35 Appendix B Figure A1. Major monoterpene constituents found in Eugenia and Syzygium essentials oils. Figure A1. Major monoterpene constituents found in Eugenia and Syzygium essentials oils. . Figure A2. Major sesquiterpene hydrocarbons found in Eugenia and Syzygium essentials oils. Figure A2. Major sesquiterpene hydrocarbons found in Eugenia and Syzygium essentials oils.  Figure A3. Major oxygenated sesquiterpenes found in Eugenia and Syzygium essentials oils. Figure A3. Major oxygenated sesquiterpenes found in Eugenia and Syzygium essentials oils.
Biomolecules 2020, 10, x FOR PEER REVIEW 30 of 35 Figure A4. Other major constituents found in Eugenia and Syzygium essentials oils.