Investigation on the vibrational and antimicrobial potential of (E)-Labda-8 (17), 12-diene-15, 16-dial from Alpinia nigra (Gaertn.) B.L. Burtt

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Alpinia nigra, a rhizomatous plant belonging to the Zingiberaceae family, grows luxuriously in North-East (NE) India. Like all members of the “ginger” family, this plant too is known for its culinary properties and home remedial uses among the ethnic tribes of NE India; however, till date, this plant has not been able to draw much attention from the scientific community. Thus, the current research work was aimed at the utilization of this medicinally important plant as source bioactive compounds of pharmacological importance. A novel method was developed and the seed-to-solvent ratio was optimized for sourcing maximum yield of seed extracts, along with an active component. Characterization of this component by HRMS, FTIR, NMR and Raman spectroscopy revealed it as a labdane-type diterpene, (E)-Labda-8(17), 12-diene-15, 16-dial. Experimental vibrational spectra were correlated with the theoretical spectra obtained at DFT level. Studies on the contact angle, optical rotation, DSC, TGA and viscosity revealed important industrially important parameters of the extracts and compound. In-silico studies revealed this compound to be unsuitable for oral administration (theoretically); hence, hemato-compatibility studies (both qualitative and quantitative) were carried out to determine its maximum suitable dosage for IV administration. Antimicrobial studies assessed by growth kinetics study, FESEM, fluorescent microscopy, cell leakage analysis, Raman spectroscopy etc. revealed the high degree antibacterial activity. The possible mode of action of this compound against Gram negative bacteria is by cell lysis and disruption but no activity was observed against Gram positive bacteria; whereas the growth of Candida albicans was temporarily inhibited in a dose dependent manner possibly by cell surface interaction, preventing substrate uptake. The hydrophobic nature of the diterpene acted as a hindrance to its biological activity in an aqueous environment, hence, nanoparticles were used, which increased the efficacy of the low, yet water soluble concentrations of the compound. At the same time, the current work also provides an interesting scientific insight on the utilization of in-house synthesized labdane diterpene loaded oil-in-water microemulsions as a promising, futuristic antibacterial agent against a wide range of bacterial pathogens, especially the antibiotic resistant strains.
Supervisor: Latha Rangan