Preparation, physicochemical characterization and computational studies of Plectranthus ornatus codd essential oil/β-cyclodextrin inclusion complex

This study aims to prepare and chemically characterize inclusion complexes (IC) between β-cyclodextrin (β-CD) and Plectranthus ornatus essential oil (OEPO). Samples were evaluated by gas chromatography coupled with mass spectrometry (GC-MS) and Fourier transform infrared spectroscopy (FTIR). Moreover, a theoretical study concerning inclusion complexes of β-CD with the four main compounds of OEPO through Molecular Dynamics (MD) simulations, Density Functional Theory (DFT) and Parametric Method-6 (PM6) calculations was performed. The GC-MS analysis of OEPO resulted in the identification of 36 volatile compounds, with the four main components of this essential oil being: β-caryophyllene (21.52%), terpinen-4-ol (6.89%), phytol (4.00%) and caryophyllene oxide (3.32%). The GC-MS results indicated the presence of OEPO volatile compounds in the ICs prepared by the water/ethanol kneading (KWE) and water kneading (KW) method. More specific, β-CD inclusion complexes with β-butoxyethanol (1.15%), 1,3,8-ρ-menthatriene (0.45%), β-caryophyllene (0.20%), phytol (0.41%) and δ-cadinol (0.20%) prepared by the KWE method were detected, while the KW inclusion method resulted in the encapsulation of β-butoxyethanol (3.98%) only. The higher inclusion content of OEPO molecules in KWE was also confirmed by FTIR, with the KWE complex showing more OEPO absorption bands than KW. The PM6 study showed that hydrogen bonds are important, but hydrophobic interactions are the main driving force for triggering the complex formation of the main OEPO volatile compounds with β-CD. The intermolecular host-guest interactions and the investigation for the most suitable inclusion mode were examined by conducting several Molecular dynamics simulations. Moreover, PM6 and DFT approaches revealed the most stable cyclodextrin inclusion complex structures. These findings are in accordance with the experimental data from GC-MS analysis. The effect of solvent on the interaction energy was also studied through simulations and the results showed that the most stable structures are characterized by the formation of hydrogen bonds between guests and cyclodextrin hosts. The developing of such preparation methods that results in ICs like OEPO/β-CD may be useful in the optimization of existing formulations and therefore, be adopted by the pharmaceutical industry