Chemical composition, antioxidant and antibacterial properties of Pteranthus dichotomus from Algerian Sahara

The phytochemical study of ethyl acetate and n-butanol extracts of Pteranthus dichotomus Forssk. led to the isolation and identification of 11 compounds, including three glycolipids 1–3, one lignan 4, three flavonoids 5–7 and four phytosterols 8–11. Structures of the isolated compounds have been elucidated by analysis of 1D and 2D NMR data, and mass spectrometry EI-MS and ESI-MS and by comparison with literature data. Furthermore, the ethyl acetate and n-butanol extracts were examined for their antioxidant and antibacterial activities. The results showed that both extracts (PDAC and PDBU) had a moderate antioxidant activity (IC50 = 375.514 μg/mL and 691.333 μg/mL) respectively.

Pteranthus dichotomus Forssk. belonging to Caryophyllaceae family is an herbaceous plant, which is also called as P. echinatus Desf. (Ozenda 1991). It is found in the northern Algerian Sahara and in the Hoggar region (Quezel & Santa 1963). In Egyptian traditional medicine, the leaves of P. dichotomus are used as an ocular antiseptic (El-Seedi et al. 2013). The aqueous extract of this species exhibited strong cytotoxicity (above 97%) against cultured melanoma cell lines (Sathiyamoorthy et al. 1999). Previous investigation on P. dichotomus has revealed the presence of flavonoids and polyphenols (Atta et al. 2013). This plant showed good antiinflammatory and moderate antipyretic effects whereas its alcoholic extract has an antitumour activity (Atta et al. 2013).
In this investigation, we report the isolation and characterisation of 11 known compounds from ethyl acetate and n-butanol extracts of P. dichotomus. Moreover, the antioxidant and antimicrobial activities of these extracts have been investigated.    2 701 Natural Product Research 6 -7 were reported from the genus Pteranthus for the first time and the compounds 1-4 were obtained for the first time from Caryophyllaceae family.

Total phenol content of P. dichotomus
The total phenolic contents of ethyl acetate and n-butanol extracts of P. dichotomus were calculated as mg gallic acid equivalent (GAE) (Folin-Ciocalteu method). The total phenolic content of PDAC extract (27.140^1.836 mg GAE/mg extract) was higher than that of PDBU (7.007^0.155 mg GAE/mg extract). This difference is mainly due to the presence of lignan 4 and flavonoids 5 and 6 in PDAC extract.

Antioxidant activity
2.3.1. Free radical scavenging ability by the use of a stable DPPH radical (2,2-diphenyl-l-1 picrylhydrazyl) Antiradical activity was evaluated by measuring the scavenging activity of P. dichotomus samples against DPPH free radical. Quercetin, used as reference, showed an antiradical activity value with IC50 of 1.149 mg/mL. The four P. dichotomus samples showed a significant ( p , 0.05) scavenging effect on the DPPH radical in a dose-dependent manner based on the calculated IC50 values presented in Table S1, the samples are ordered for their scavenging activity as follows: compound (7) (358.888 mg/mL) . PDBU (375.514 mg/mL) . PDAC (691.333 mg/mL) . Fr (4þ5).5 (912.667 mg/mL). The subfraction Fr (4þ5).5 of PDBU containing glycolipids as major compounds displayed a weak activity in comparison with the other extracts. However, the antioxidant capacities of glycolipids may be due to the composition and proportion of mono-and polyunsaturated fatty acids (Kitamoto et al. 2002;Alejandro et al. 2011).

b-Carotene bleaching assay
Antioxidant activity of P. dichotomus samples was also estimated by bleaching of b-carotene/ linoleic acid emulsion system. Antioxidant activity of samples (PDBU, PDAC, 7 and Fr (4þ5).5 ) is increased in the course of time. All samples have lower antioxidant activity than BHT used as standard. The highest antioxidant activity among the samples was observed for PDBU (71.48%) where Fr (4þ5).5 has the lowest antioxidant activity (47.99%) ( Figures S1 and S2). This difference could be explained by the richness of PDBU with polyphenol compounds. The b-carotenelinoleate model is similar to an oil-in-water emulsion system, and variations in activities could be attributed to differences in the proportion of hydrophobic and hydrophilic compounds present in each extract (Wijeratne et al. 2006).

Antibacterial activity assay
Extracts of P. dichotomus were tested against five bacterial strains (Staphylococcus aureus ATCC 25923, Pseudomonas aeruginosa ATCC 27853, Escherichia coli ATCC 25922, Klebsiella pneumoniae BLSE and Enterobacter sp. BLSE). The results given in Table S2 showed that, in general, PDBU and PDAC extracts possessed a moderate antibacterial activity. S. aureus and Enterobacter sp. were the most sensitive microorganisms to the PDAC extract but it did not exhibit any antibacterial activity against K. pneumonia and P. aeruginosa. The PDBU extract inhibited only the growth of Gram-negative bacteria strains E. coli and K. pneumoniae indicating zone of inhibition values of 10 and 13.5 mm at 0.5 g/mL and 8 mm at 0.25 g/mL, respectively.

Conclusion
According to literature data, all the isolated compounds 1-4 and 6 -7 are found for the first time in the genus Pteranthus. To the best of our knowledge, compounds 1-4 were obtained from the family Caryophyllaceae for the first time. Further phytochemical studies should be carried out to investigate the fractions of n-butanol extract containing particularly flavonoids and lignans. From this study we can conclude that P. dichotomus showed moderate antibacterial and antioxidant activities.

Supplementary material
General experimental methods, spectra and NMR data relating to this paper are available online, alongside Tables S1-S2 and Figures S1-S2.

Disclosure statement
No potential conflict of interest was reported by the authors.