Chemical composition, in vitro antioxidant and antibacterial activities of Centaurea resupinata subsp. dufourii (dostál) greuter

Abstract The current study focuses on the chemical composition, and evaluation of antioxidant and antibacterial activity of the aerial parts of Centaurea resupinata subsp. dufourii. Using different chromatographic methods nine compounds 1–9 were isolated. The structural identification of isolated compounds was achieved using several spectroscopic methods NMR techniques (1H NMR, 13C NMR, COSY, HSQC, HMBC) and mass spectroscopy (ESI-MS) and by comparison with literature data. The structures of these compounds were identified as nicotiflorin (1), apigetrin (2), chrysoeriol (3), apigenin (4), chrysin (5), daucosterol (6), β-sitosterol (7), taraxastrerol (8) and lupeol (9). The antibacterial and antioxidant activities of ethyl acetate and n-butanol extracts have been evaluated. The antioxidant activity was assessed in vitro using DPPH radical scavenging method, which showed that ethyl acetate extract possessed an interesting antioxidant potential (IC50 = 36.263 ± 0.005 μg/mL). Graphical Abstract

The present paper deals with the isolation and structure elucidation of nine known metabolites Figure 1 from the ethyl acetate extract of aerial parts of C. resupinata subsp. dufourii. Since several isolated compounds were described as possessing an interesting antioxidant activity, we found it pertinent to evaluate the antioxidant activity of the ethyl acetate and n-Butanol extracts. In addition, both extracts exhibited a moderate potential as antibacterial compounds against micro-organisms.
To the best of our knowledge, this work constitutes the first phytochemical and biological study of C. resupinata subsp. dufourii.

Total phenol and flavonoid contents
The concentration of total phenolic content (TPC) and flavonoid (TFC) in various extracts of aerial parts of C. resupinata subsp. dufourii were measured using Folin-Ciocalteu method, and the aluminium chloride method respectively. The results were found to be 173.3 ± 4.321 mg GAE/mg and 44.223 ± 3.052 mg QE/mg, (Table S1, Supplementary material).
This finding could be explained by the present phytochemical investigation of ethyl acetate extract witch revealed the presence of many flavonoids compounds.

Evaluation of antioxidant activity
The antioxidant activity of aerial parts of ethyl acetate and n-butanol extracts of C. resupinata subsp. dufourii was measured in vitro using the 1,1-diphenyl-2-picrylhydrazyl (DPPH) according to the procedure described by (Hatano et al. 1988). Ascorbic acid was used as the reference compound. Ethyl acetate extract showed the highest activity of radical scavenging in comparison with n-butanol extract (IC 50 ¼ 36.263 6 0.005 lg/ml; IC 50 ¼ 97.013 ± 0.019 lg/ml) (Table S2, Supplementary material), which can be explained by its richness in flavonoids components, known especially for their antioxidant activity.

Evaluation of antibacterial activity
In vitro study of antibacterial activity of ethyl acetate and n-butanol extracts of aerial parts of C. resupinata subsp. dufourii, was assessed using the disc diffusion method described by (Kursat and Erecevit 2009) with sight modifications, five bacterial strains including one Gram positive bacteria: Staphylococcus aureus ATCC 25923, and four Gram negative bacteria: Pseudomonas aeruginosa ATCC 27853, Escherichia coli ATCC 25922, Proteus mirabilis, and Klebsiella pneumonia BLSE. Obtained results given in (Table S3, Supplementary material) showed that the two extracts possessed a moderate antibacterial activity. Both were active against all test strains except K. pneumoniae and P. aeruginosa ATCC 27853, and the highest diameter of inhibition were was appeared on P. mirabilis with 15.65 ± 0.283 mm for the n-butanol extract.

Conclusion
The phytochemical investigation of the ethyl acetate extract of aerial parts of C. resupinata subsp. dufourii led to the isolation and characterization of nine known compounds five flavonoids (1-5), two triterpenes (6-7) and two phytosterols (8-9). Ethyl acetate extract showed high phenolic and flavonoid contents. This result was in good agreement with their antioxidant activity. Moreover, both ethyl acetate and n-butanol extracts of aerial parts of C. resupinata subsp. dufourii have moderate potential as antibacterial compounds against micro-organisms. Shoeb M, Jaspars M, MacManus SM, Celik S, Nahar L, Kong-Thoo-Lin P, Sarker SD. 2007.
Anti-colon cancer potential of phenolic compounds from the aerial parts of Centaurea gigantea (Asteraceae). J Nat Med. 61 (2)