Chemical composition of the essential oils of selected aromatic South African plants and assessment of their biological activities for cosmetic uses

Abstract

The South African flora is endowed with botanical materials used for skin therapies in traditional practices. Nowadays, because the organic cosmetics consumerism is favoured, there is a need to explore the essential oils of indigenous plants for sought-after cosmeceutical properties. The first objective of this study was to, investigate the chemical makeup of the essential oils (EOs) of Oncosiphon suffruticosum (L.) Källersjö, Helichrysum petiolare Hilliard & B.L.Burtt, Helichrysum cymosum (L.) D.Don subsp. cymosum, Helichrysum odoratissimum (L.) Sweet, Salvia aurea L. (syn. Salvia africana-lutea L.) and Salvia chamelaeagnea P.J.Bergius by chromatographic and spectroscopic techniques. The second objective was to determine their antimicrobial, antioxidant, photoprotective, and antityrosinase properties in vitro. Gas chromatography-mass spectrometry (GC-MS) analyses of Oncosiphon suffruticosum EO revealed the major constituents: (+)-2-bornanone (31.21%) and filifolone (13.98%). Helichrysum petiolare EO contained faurinone (20.66%) and (E)-β-ocimene (17.21%). Faurione was structurally elucidated by 1H and 13C nuclear magnetic resonance (NMR). Helichrysum odoratissimum EO was found to contain chiefly 1,8-cineole (17.44%), α-pinene and γ-curcumene (15.76%). Helichrysum cymosum EO was found to be made up of α-pinene (29.82%) and (E)-caryophyllene (19.20%). Salvia aurea EO was found to contain primarily epi-α-cadinol (14.24%) and (E)-caryophyllene (7.93%), and Salvia chamelaeagnea, limonene (28.00%) and viridiflorol (13.42%). The antimicrobial activity of the essential oils (25.6–0.2 mg/mL) was assessed via the broth microdilution test for the determination of MICs against three skin pathogenic bacteria, S. aureus, P. aeruginosa, and E. coli. According to the results, S. aurea EO was found to display the lowest MIC of 6.4 mg/mL against S. aureus, followed by S. chamelaeagnea, O. suffruticosum, and H. odoratissimum EOs at 12.8 mg/mL. H. petiolare and H. cymosum EOs were found inactive (MIC > 25.6 mg/mL) over the concentration range against this organism. P. aeruginosa was found to be most susceptible to O. suffruticosum EO at 6.4 mg/mL and to all other oils at 12.8 mg/mL except for S. chamelaeagnea EO which was found to be inactive for this organism. For E. coli, all the essential oils exhibited the same activity with MIC at 12.8 mg/mL. Ampicillin, the positive control, had an MIC < 0.2 mg/mL against S. aureus and E. coli whereas P. aeruginosa was found to be resistant against it. The antioxidant capacities of the EOs were evaluated by the 2,2-diphenyl-1-picrylhydrazyl (DPPH), 2,2′-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid) (ABTS), ferric reducing antioxidant power (FRAP), and oxygen radical absorbance capacity (ORAC) assays. The oils were tested at 2–0.5 mg/mL in the DPPH and ABTS assays and at 2 mg/mL in the FRAP and ORAC assays. In the DPPH assay, S. chamelaeagnea EO exhibited the highest percentage radical scavenging activity (% RSA) over the concentration range as 26.86 ± 0.10%–8.83 ± 0.68% whereas Trolox® positive control gave 94.94 ± 0.02%–94.45 ± 0.04%. In the ABTS assay, O. suffruricosum EO exhibited the highest % RSA over the concentration range as 87.17 ± 0.76%–71.46 ± 0.04% giving TEAC values (% RSD) of 9431.2(0.9%)–7750.1(0.1%) μmol TE/L (2-0.5 mg/mL respectively). This was followed closely by H. petiolare EO with respective % RSA and TEAC values of 84.42 ± 0.43% and 9131.4(0.5%) at 2 mg/mL, and 67.08 ± 0.76% and 7281.7(1.1%) μmol TE/L at 0.5 mg/mL. Gallic acid used as a positive control gave % RSA of 97.97 ± 0.13%–98.05 ± 0.03% and 605840(4.6%)–195220(3.2%) μmol TE/L over the concentration range (2–0.5 mg/mL). In the FRAP assay, H. odoratissimum EO had the highest value of 3026.6(6.1%) μmol AAE/L (% RSD) and gallic acid positive control gave 635500(0.6%) μmol AAE/L (% RSD). In the ORAC assay, the essential oils performed near equal with ORAC values ranging from 6701.8(0.9%) to 6549.7(1.5%) μmol TE/L (% RSD) while (-)-epigallocatechin gallate (EGCG) positive control was found as 26904(1.22%) μmol TE/L. Thin-layer chromatography-direct bioautography (TLC-DB) was conducted to discover radical scavenging materials present in the essential oils. The results showed that H. petiolare EO contained two phenolics, eugenol and 7-hydroxycadalene. The photoprotective property of the essential oils was evaluated by spectrophotometric determination of the sun protection factor (SPF) over the UV range of 290-320 nm. The results revealed that the essential oil of O. suffruticosum possessed the highest SPF value as 2.299 followed by H. petiolare essential oil as 1.511. H. cymosum, S. chamelaeagnea, H. odoratissimum, and S. aurea essential oils exhibited SPF values below one of 0.956, 0.391, 0.309, and 0.216, respectively. The tyrosinase inhibitory activities of the essential oils were determined using mushroom tyrosinase. The absorbance of L-3,4-dihydroxyphenylalanine (L-DOPA) was monitored at λ490 nm using L-tyrosine as a substrate. The essential oils samples and Kojic acid positive control were tested at 200 μg/mL and 50 μg/mL. The results showed that the essential oils were found to exhibit close inhibitory activities of 63.30 ± 2.35–51.53 ± 10.30% (H. odoratissimum EO–S. chamelaeagnea EO) and 28.62 ± 0.30–19.13 ± 0.81% (H. odoratissimum EO–S. aurea EO) at 200 μg/mL and 50 μg/mL, respectively. Whereas the inhibitory activities of the Kojic acid were found to be 96.24 ± 3.62% and 98.34 ± 0.80% at 200 μg/mL and 50 μg/mL, respectively.