A Study of Karabaghian Skullcap (Scutellaria platystegia Juz.): Antioxidant and Antibacterial Activity Assays, Essential Oil Analysis, and Isolation of Its Phenolic Compounds

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Background: Scutellaria platystegia Juz. is an herbaceous perennial, distributed in the northwest of Iran and southern Caucasian countries? Objectives: This study aimed to examine the antioxidant and antibacterial activities of the aerial part of S. platystegia and to determine the phytochemical constituents of its extract and essential oil. Methods: The ferric reducing ability of plasma (FRAP) and 2,2-diphenyl-1-picrylhydrazyl (DPPH) methods were used to examine the antioxidant effects of fractions obtained from the hydroalcoholic extract of the aerial part of S. platystegia. The antibacterial activity screening was also performed using the disk diffusion and microdilution methods. The phytochemical constituents of the most active fractions were investigated using column chromatography. Nuclear magnetic resonance (NMR) imaging and UV-Vis spectral analysis were used to determine the chemical structure of the isolated compounds. The GC-MS technique was also employed to identify the essential oil composition of the plant. Results: In the antioxidant activity assay, the n-butanol fraction was the most potent fraction, with a half maximal inhibitory concentration (IC50) of 16.14 ± 0.8 µg/mL on the DPPH assay and 736.4 ± 4.6 mmol FeSO4.7H2O equivalent per gram of sample on the FRAP assay. The n-butanol fraction exhibited very strong antibacterial activities against Staphylococcus aureus, Staphylococcus epidermidis, and Shigella dysenteriae (inhibition zone diameter: 20 - 25 mm, MIC: 125 - 250 µg/mL). The phytochemical analysis of the most active fractions resulted in the isolation of the following components from the chloroform fraction: 5,7,2’-trihydroxy-8,6’-dimethoxyflavone; 5-hydroxy-6,7,8,2’,6’-pentamethoxyflavone; 5,2’,6’-trihydroxy-7,8-dimethoxyflavone; 5,2’,6’-trihydroxy-6,7,8-trimethoxyflavone; and 5,4’-dihydroxy-7-methoxyflavone. Besides, the following components were extracted from the n-butanol fraction: luteolin-7-O-β-D-glucopyranoside; verbascoside; apigenin; kaempferol; caffeic acid; rosmarinic acid; apigenin-7-O-β-D-glucopyranoside; apigenin-7-O-β-D-(-6’’-(E)-caffeoyl)-glucopyranoside; and luteolin. Fourteen compounds were also identified in the plant essential oil; terpinen-4-ol (44.41%), α-terpineol (10.75%), caryophyllene oxide (9.61%), and thymol (8.73%) were the main compounds. Conclusions: This study introduced S. platystegia as a plant rich in bioactive phenolic compounds, with considerable antioxidant and antimicrobial activities. Therefore, it can be suggested as a potential candidate to be evaluated in future biological studies.