Encapsulation in liposomes has always been an efficient approach/strategy to improve the stability of sensitive bioactive compounds such as essential oils (EOs) and to increase their biological activities. However, the stability of liposomal formulation remains a major concern for the control of the delivery of encapsulated ingredients. Given the stability that cholesterol (Chol) confers to the membrane, the Chol content used for the preparation of liposomes encapsulating the hydrophobic EO components has to be carefully chosen. In this work, we investigated the effect of Chol content on the permeability of liposome membranes induced by various EO components. Blank liposomes made of DPPC and Chol at different DPPC/Chol molar ratios (100:10; 100:25; 100:50; 100:75; 100:100) were prepared and the final composition of the membrane was determined after extrusion. The same formulations encapsulating the hydrophilic fluorescent agent, sulforhodamine B (SRB), were also prepared and exposed to the EO components at a molar ratio of DPPC:EO of 100:25. The membrane permeability was monitored by following the release of SRB from liposomes with time at 37 °C. Results showed that the DPPC:Chol molar ratios experimentally determined for the formulations exceeding 10% Chol were lower than the theoretical values. Among the 22 EO components tested, 13 molecules displayed a significant permeabilizing effect on 10% Chol liposome membranes. Most of these possess a hydroxyl group. The EO induced permeability was also dependent on the Chol content which in turn affects the membrane phase: the EO components effect was reduced upon increasing Chol content from 10 to 21% Chol (S0-L0 phase) keeping 9 molecules effective on the 21% Chol membranes. At high Chol content where the membrane is in the L0 phase, only five EOs components exerted a permeabilizing effect (menthol, eugenol, thymol, guaiacol and linalool) on 40% Chol membranes with no effect observed at 43% and 55% Chol. The EO's effect was also linked to the hydrophobicity of the molecule: hydrophobic compounds having no hydroxyl group showed a weak permeabilizing effect only in the So-Lo phase. Conversely, when the hydrophobicity of EO component having a hydroxyl group increases, their effect persisted up to higher Chol content. Consequently, the DPPC:Chol ratio of the formulation is chosen taking into account the structure of the compound, its hydrophobicity and its effect on the permeability at different Chol content: a formulation comprising 40% Chol is suggested for highly hydrophobic molecules whereas a formulation with higher Chol content (more than 40%) could be selected for less hydrophobic compounds.
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