Phytochemical Investigation, Antibacterial, DPPH Radical Scavenging Activities, and In silico Molecular Simulation Studies of Constituents of Selected Medicinal Plants of Ethiopia

Abstract

People all over the world have been suffering from both communicable and noncommunicable diseases, including HIV, malaria, tuberculosis, hepatitis virus, cardiovascular disease, cancer, chronic respiratory diseases, and diabetes. Millions of people die from these health threat diseases each year. Lack of access to essential medicines and drug resistance are among the reasons why these diseases have not yet been eradicated from the globe. The use of medicinal plants is believed to alleviate issues related to access to essential medicines and combat drug resistance. Clematis hirsuta, Grewia ferruginea, Rhus ruspolii, Olinia rochetiana, and Rubus apetalus are among the plants traditionally used to treat different diseases in Ethiopia. However, the phytochemical constituents and the biological activities of these plants are not well documented. Therefore, this study aimed to isolate phytochemicals from the root of C. hirsuta, the bark of G. ferruginea, and the leaves of R. ruspolii, O. rochetiana, and R. apetalus and evaluate their in vitro and in silico antibacterial and DPPH scavenging activities. GC-MS analysis of the essential oil from G. ferruginea and non-polar fractions from R. ruspolii, O. rochetiana, and R. apetalus furnished 58 different compounds. Chromatographic separation and NMR spectroscopic techniques identified a total of 28 different compounds. Oleic acid (188), palmitic acid (145), stigmasterol (70), β-sitosterol (17), boehmenan (189), erythro carolignan E (190), and threo carolignan E (191) were identified from the root extract of C. hirsuta. Lupeol (16), β-sitosterol (17), stigmasterol (70), 7-keto-β-sitosterol (192), 6-ketositostanol (193, new to the genus), daucosterol (42), catechin (65), and epicatechin (69) were identified from the stem bark of G. ferruginea. Palmitic acid (145), 3,4-dihydroxybenzoic acid (194), myricetin (55), cupressuflavone (195), amentoflavone (62), shikimic acid (196), avicularin (197), and myricetin-3-O-5′′-acetylarabinofuranoside (198, new to the genus) were identified from the leaves of R. ruspolii. Ursolic acid (41), 5-hydroxy-4-methyl-5,6-dihydro (2H)-pyran-2-one (199), quercetin-3-O-(1-D-galactopyranoside) (200), olinioside (84), and 4-O-β-D-glucopyranosylcaffeic acid (83) were identified from the leaves of O. rochetiana. Compounds 41 and 200 are new to the genus, whereas compound 199 is a new natural product. Palmitic acid (145), palmityl alcohol (201), β-sitosterol (17), tiliroside (91), and astragalin (202) were identified from the leaves of R. apetalus. 100 mg/mL extracts and fractions and 5 mg/mL essential oil and isolated compounds showed inhibition zones (IZs) ranging from 7.93 ± 0.65 to 16.33 ± 0.47 mm against all tested bacteria compared to ciprofloxacin (0.5 mg/mL), which ranged from 18.42 ± 0.26 to 22.18 ± 0.22 mm. The highest IZ (16.33 mm) was exhibited by the DCM:MeOH (1:1) extract of the leaves of R. ruspolii against Escherichia coli. Whereas the lowest IZ (7.93 mm) was shown by the EtOAc fraction of G. ferruginea stem bark against Pseudomonas aeruginosa. Of all the isolated compounds, Staphylococcus aureus was the most susceptible to compound 197 (IZ: 14.67 ± 0.47 mm) and least susceptible to compound 196 (IZ: 8.33 mm). Streptococcus pyogenes was the most susceptible to compound 17 (IZ: 15.07 ± 0.74 mm) and least susceptible to compound 196 (IZ: 8.67 ± 0.50 mm). E. coli was the most susceptible to compound 195 (IZ: 12.67 ± 0.50 mm) and least susceptible to compound 196 (9.00 ± 0.82 mm). P. aeruginosa was the most susceptible to compound 16 (IZ: 15.20 ± 0.28 mm) and least susceptible to compound 196 (IZ: 8.67 ± 0.47 mm). The results of in vitro antibacterial activity evaluation conducted against E. coli and P. aeruginosa were supported by molecular docking studies against their target enzymes (E. coli DNA Gyrase B and Pseudomonas quinolone signal A (PqsA), respectively. Among the isolated compounds, at 62.5 µg/mL, compound 189 exhibited the highest DPPH radical (92.1%), while compound 17 exhibited the least (18.50%) compared to the positive control ascorbic acid (96.9%). From the in silico druglikeness and toxicity prediction, most of the isolated compounds fulfilled Lipinski’s rule of five. They were predicted to have less toxicity compared to the standard drugs used in this study. Generally, the extracts, fractions, essential oils, and isolated compounds from the studied plants showed promising antibacterial and DPPH radical scavenging properties. Most of the isolated compounds have good permeability and solubility in the human body, and none of the isolated compounds are fatal if swallowed. Therefore, these findings support the traditional use of these medicinal plants to treat different bacterial diseases. However, both in vitro and in vivo qualitative, quantitative, pharmacokinetic, and pharmacodynamic investigations are very important to standardize the use of these medicinal plants.