Chemical Composition, Antibacterial and Antioxidant Activities of the Essential Oil of Psychotria serpens

Published in Khimiya Prirodnykh Soedinenii, No. 4, July–August, 2020, pp. 643–644.

Psychotria serpens L., an evergreen epiphytic or terrestrial liana belonging to the family Rubiaceae, is widespread throughout tropical and subtropical Africa, America, Asia, Madagascar, and Pacific Islands [[1]]. In Chinese folk medicine, the whole herbs of P. serpens are used as an antirheumatic, analgesic, muscles-relaxing, and circulation-promoting drug [[2]], and are also used to treat colds, fever, and asthma [[3]]. In the previous phytochemical investigation on the whole plant of Psychotria serpens L., one new type of glycosylsphingolipids, psychotramide A–D, has been identified [[4]]. Recently, several flavonoids and flavone glycosides, including quercetin, kaempferol, sevenetin, rutin, kaempferol-3-O-glucoside, tamarixetin-3-O-rutinoside, etc., were also isolated from this plant [[5]]. In the present study, the chemical composition and in vitro antimicrobial and antioxidant activities of essential oil (EO) from P. serpens are reported for the first time.

The fresh P. serpens was collected in September 2018 from Guigang in Guangxi Province of China. A voucher specimen (No. 0180017) was deposited in the Laboratory of Botany of Marine College, Shandong University. The aerial parts of the fresh P. serpens (300 g) were hydrodistilled for 3 h using a Clevenger apparatus. The distilled oil was obtained using ethyl ether as a collecting solvent and dried over anhydrous sodium sulfate and stored at 4°C until analyses. The yield of the oil was 0.05% (w/w) based on the fresh weight of the sample.

The essential oil was analyzed by GC-FID and GC-MS methods as described previously [[6]]. Identification of the components was carried out by matching their mass spectra with NIST 14 MS Search 2.2 Mass Spectral Database and comparing their Kovats retention indices from the HP-5MS column (relative to C8–C30n-alkanes, under the same experimental conditions) with reference libraries [[7]]. The relative amounts of individual components were calculated based on GC peak area without using a correction factor.

A total of 62 compounds were identified, which represent 96.5% of the total oil (Table 1). The essential oil of P. serpens was characterized by the presence of monoterpenes (14.3%), sesquiterpenes (29.8%), and oxygenated sesquiterpenes (21.6%). The main constituents were 1-octen-3-ol (23.2%), linalool (10.0%), neointermedeol (7.5%), valencene (7.1%), methyl salicylate (7.1%), α-selinene (6.0%), and β-cedrene (4.7%).

Chemical Composition of the Essential Oil of P. serpens

aCompounds are listed in order of their elution from a HP-5MS column; b Linear retention index relative to C8–C30n-alkanes on HP-5MS column.

The antibacterial activities of the oil were assessed against three Gram-positive and two Gram-negative bacterial strains using the disc agar diffusion [[9]] and microdilution methods [[10]]. The results are shown in Table 2. The essential oil exhibited significant antibacterial activity against S. aureus (MIC = MBC = 0.039 mg/mL) and moderate antibacterial activity against B. subtilis, P. larvae, and P. aeruginosa (MICs: 0.156–0.312 mg/mL). However, the oil showed poor antibacterial activity against E. coli (MIC = 1.250 mg/mL).

Antibacterial Activity of Essential Oil of P. serpens

The diameter of the inhibition zones (mm), including the disc diameter (6 mm), are given as the mean ± SD of triplicate experiments. a Diameter of the inhibition zones of the EO: essential oil of P. serpens (tested volume, 1 mg/mL); positive control: Ch, chloramphenicol (tested volume, 0.01 mg/mL).

The multiple methods of DPPH radical scavenging activity [[11]], ABTS·+ scavenging activity [[12]], and FRAP assays [[13]] were used to evaluate the antioxidant capacities of EO. The results are presented in Table 3. It was shown that the P. serpens oil exhibited moderate ABTS·+ scavenging activity with an IC50 value of 0.438 mg/mL and moderate ferric ion reducing activity with a TEAC (Trolox equivalent antioxidant concentration) value of 46.31 μmol Trolox × g–1, while the oil showed weak DPPH radical scavenging activity (IC50 value of 1.250 mg/mL).

Results of Antioxidant Activity in vitro (DPPH, ABTS, and FRAP) of the Essential Oil of P. serpens

aIC50 = the concentration of compound that affords a 50% reduction in the assay; b positive control used.