Mechanism of β–cyclodextrin - thyme nanocomplex formation and release: In silico behavior, structural and functional properties

Abstract

Nanoencapsulation of thyme essential oil (TEO) volatiles into β-cyclodextrin (β-CD) and their release requires in-depth kinetic, structural and in silico studies for elucidating the mechanisms for attaining the equilibrium state. Monoclinic crystalline structures of β-CD/TEO with 5 –10 μm were formed by TEO encapsulation and visualized using SEM. The β-CD:TEO 8.5:1.5 (w/w) ratio ensures 71.66% TEO retention efficiency and a cumulative release in hexane after 7 h of 75%. Thymol molecule is the favourite candidate to occupy the inner cavity of β-CD, although competition by carvacrol, o-cymene, 2-carene and camphene against thymol was noticed by GC/MS when increasing the TEO concentration. The molecular mechanics calculations suggested that binding of thymol, carvacrol and o- cymene within the hydrophobic cavity of β-CD is thermodynamically achievable, having complexation energy between -18.48 ÷ -22.07 kcal/mol, nonetheless the most stable complex is formed with α- pinene, if energy input is provided, displaying the lowest complexation energy, -25.89 kcal/mol. All inclusion complex (IC) powders had similar antiradical activity against DPPH (40 - 43%). The IC2 β-CD/TEO 8.5:3.00 (w/w) powder displayed antibacterial activity against Bacillus cereus, while IC3 β-CD:TEO 8.5:1.50 (w/w) exhibited the strongest antifungal activity of all powders against Geotrichum candidum and Rhodotorula glutinis. The ICs obtained suggest a competitive mechanism of TEO components in occupying the β-CD inner cavity that predispose each powder formulae for specific applications in food in relation with their functional properties.