Crystal structure and antibacterial evaluation of epifriedelinol isolated from Vitex peduncularis Wall. ex Schauer

1 Introduction

The genus Vitex belongs to the family Verbenaceae and its species are shrubs or trees, present in the tropical and temperate regions of the world. In India 13 species of Vitex are reported (Rajendran and Daniel, 2002). Vitex peduncularis “Mayiladi” or “Malainochi” in Tamil, is a tree about 18 m tall. The bark of this plant is used to cure pains in the chest. Leaves, root bark or young stem bark are used traditionally to cure malarial fever (Kirtikar and Basu, 1991). In the traditional medicine of Mizoram, India, leaf, root or bark decoction is orally administered in the treatment of malaria (Sharma et al., 2001).

Previous phytochemical studies showed that the plant, V. peduncularis is known to be a source of flavonoids, iridoids, triterpenoids and flavonol methyl ether (Sharma, 1955; Rao and Venkateswarlu, 1962; Sahu et al., 1984; Suksamrarn et al., 2002; Rudrapaul et al., 2014). In the present study, Isolation of epifriedelinol (CAS# 16844-71-6) from the leaves of V. peduncularis and the crystal structure and molecular and its antibacterial activity of the compound are reported. Epifriedelinol is very abundant in nature (Sainsbury, 1970) and also has been reported from plants (Rao et al., 1990; Menelaou et al., 1992; Cambie et al., 1992). But, the detailed crystal data and antibacterial activity have not been reported previously. To the best of our knowledge, the presence of epifriedelinol in V. peduncularis and its antibacterial evaluation are reported for the first time.

2 Materials and methods

3 Results and discussion

One hundred grams of dried leaf powder of V. peduncularis was extracted with 300 ml × 3 methanol in a Soxhlet apparatus for 72 h. The yield of methanol extract was 3.9%. The compound, epifriedelinol (Fig. 1) was identified by ¹H NMR, ¹³C NMR and single crystal XRD. The ¹H NMR spectrum showed that the proton under the hydroxyl residue (H-3) resonates at 3.75 ppm and appears as a multiplet. The J-value of the entire signal (5.5 Hz) indicated that the hydroxyl group should be β-oriented, because the α-oriented proton H-3, adopts the equatorial position and in split by two axial (H-2α and H-4α) and an equatorial proton (H-2β). The methyl group at C-4 appears as a doublet (integral corresponds to three protons) centred at 1.01 ppm. The spectrum also reveals seven singlets at 0.88, 0.93, 0.95, 1.00, 1.18, 0.94 and 0.98 ppm belong to the methyl protons at C-24, C-25, C-26, C-27, C-28, C-29 and C-30 respectively. The other 26 CH protons gave unresolved absorption at 1.20–1.59 ppm. The hydroxyl proton (OH proton) at C-3 appears at 2.19 ppm. The ¹³C NMR spectrum of the compound indicated the presence of 30 carbon atoms (8 methyls, 11 methylenes, 5 methines and 6 quaternary carbons). Because of the electron with drawing oxygen atom, C-3 resonates at 72.8 ppm and the signal around 49.2 ppm belongs to methine carbon at C-4. The methyl group (C-23) at C-4 appears at 11.4 ppm. Other methyl carbons at C-24, C-25, C-26, C-27, C-28, C-29 and C-30 appear at 16.4, 18.6, 20.1, 18.2, 32.1, 35.0 and 31.8 ppm, respectively. The quaternary carbons at C-5, C-9, C-13, C-14, C-17 and C-20 resonate at 37.5, 36.1, 37.9, 39.1, 29.6 and 27.8 ppm. Obviously the remaining signals at 15.8, 35.2, 42.8, 17.5, 35.7, 29.3, 30.0, 35.6, 35.3, 32.3 and 38.9 ppm belong to methylene carbons at C-1, C-2, C-6, C-7, C-11, C-12, C-15, C-16, C-19, C-21 and C-22 carbons, respectively. The signals at 53.2, 61.0 and 42.4 ppm belong to C-8, C-10 and C-18 carbons, respectively.

Close

Figure 1 (a) Molecular structure and (b) crystal structure of epifriedelinol.

Export to PPT

The crystal structure of epifriedelinol was studied by single crystal XRD and the details of crystal data and structure refinement for epifriedelinol are given in Table 1. The data collected for the crystal have been deposited at Cambridge Crystal Deposition Centre (CCDC), UK (No. 686802). All the bond lengths and angles are normal. The OH group at C-3 is axial and the methyl group, C-23 at C-4 is equatorial. The repulsion between the axial methyl groups which are at C-5 and C-14 causes a marked bowing of the rings A, B and C.

The results indicated that epifriedelinol produced (Table 2) zones of inhibition ranging from 14.00 ± 0.58 to 25.66 ± 0.88 mm to all the Gram positive and Gram negative bacteria. The MIC values were ranged between 6.25 and 50 μg/ml and the MBC values were ranged between 12.5 and 100 μg/ml. The highest activity for epifriedelinol was recorded against S. aureus (zone of inhibition = 25.66 ± 0.88 mm, MIC = 6.25 and MBC = 12.5 μg/ml), while the lowest activity was recorded against K. pneumoniae. The compound produced a mean zone of inhibition of 14.00 ± 0.58 mm, MIC value of 50 μg/ml and MBC value of 100 μg/ml against K. pneumoniae. The positive control, produced zones of inhibition between 27.33 ± 0.58 and 35.66 ± 0.33 mm against all the bacterial strains tested. The negative control, 5% DMSO had not exhibited any activity against the tested bacterial strains. The Gram-negative bacteria were found to be less susceptible whereas, the Gram-positive bacteria were highly susceptible to the compound. Mathabe et al. (2008) isolated a triterpenoid, D-friedoolean-14-en-oic acid (3-acetylaleuritolic acid) from Spirostachys africana and it showed MIC values of 50 μg/ml against S. aureus, 100 μg/ml against S. flexneri and above 200 μg/ml against S. sonnei. Another triterpenoid, lupeol isolated from S. africana showed MIC values above 200 μg/ml against these bacteria. These values are higher than the MIC values of the present study with epifriedelinol i.e., 6.5 μg/ml. D-friedoolean-14-en-oic acid (3-acetylaleuritolic acid) and epifriedelinol had the similar MIC values (50 μg/ml) against E. coli. The mechanism of antimicrobial action of terpene is reported to be closely associated with their lipophilic character. Triterpenes influence membrane structures which increase membrane fluidity and permeability, changing the topology of membrane proteins and inducing disturbances in the respiration chain (Paduch et al., 2007).

4 Conclusion

The problem of antibacterial resistance could be better understood by studying increase in microbial resistance in relation to antibiotic consumption. However, the discovery of new antibiotics especially from traditional medicinal plants might solve the problem as the existing antibiotics are less effective. In this context the epifriedelinol isolated from a traditional medicinal plant V. peduncularis has antibacterial activity against a range of bacterial species studied. Hence, epifriedelinol could be used as a natural product against infections caused by the studied bacteria. However, a detailed pharmacological investigation of this compound in in vivo is necessary.