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A bicelle, which is a bilayer molecular assembly, can be prepared by fluidizing a vesicle in the presence of a detergent. We investigated the effect of two different detergents, 3-[(3-cholamidopropyl)dimethylammonio]-2-hydroxypropane sulfonate (CHAPSO) and Triton X-100 (TX), on the formation of a bicelle from a vesicle containing oleic acid (OA) and the detergent molecules. The fluidization effect of the detergent was evaluated using the membrane packing density, which we measured using the fluorescent probe method with Laurdan, in conjunction with transmission electron microscopy to examine the morphology of the prepared bilayer molecular assemblies. As a result, it was discovered that the OA/CHAPSO system could form a heterogeneous phase with the highest packing density, implying that CHAPSO was the better detergent for a bicelle preparation, whereas the OA/TX system formed a disordered phase with the lowest packing density.

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The vesicle was designed with molecules that easily permeate the skin, and it contained unsaturated lecithin mixed with promoting agents (e.g., sodium oleate, sodium lauryl ether sulfate, and propylene glycol) 2) . In addition, Jiang et al. reported a vesicle (made of soy lecithin, polyoxyethylene sorbitan monooleate, and sodium deoxycholate) -embedded hydrogel that contained paclitaxel for the chemotherapy of melanoma 6) . These vesicles were designed with fluid bilayer membranes to achieve efficient permeation. However, the internal water phase of the vesicles was not required because docetaxel and paclitaxel are hydrophobic drugs.A bicellar mixture is mostly a nonspherical molecular assembly formed via a long-chain phospholipid bilayer membrane that is fluidized using short-chain phospholipids or detergents 7−9) . Bicellar mixtures have different morphologies, such as disk-and worm-like micelles, depending on its composition. However, most of these morphologies do not have an internal aqueous phase 7) . Thus, compared to vesicles, bicellar mixtures are more efficient drug carriers with better permeation. Bicellar mixtures with a bilayer membrane of 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) fluidized using a 1,2-dihexanoyl-sn-glycero-Abstract: Oleic acid/3-[(3-cholamidopropyl)dimethylammonio]-2-hydroxypropane sulfonate (OA/CHAPSO) bicellar mixtures are potential functional membrane materials. The lipid transfer during the formation of these bicellar mixtures was evaluated. The OA/CHAPSO bicellar mixture was prepared by mixing a solution of OA vesicles as a source of bilayer membranes with a solution of CHAPSO as a detergent at different composition ratios, x OA (= [OA] / ([OA] + [CHAPSO])). The lipid transfer was evaluated based on the leakage of fluorescent probe molecules, i.e., carboxyl boron-dipyrromethene (BODIPY C12), from the OA bilayer membranes. Mixing the CHAPSO solution with the OA vesicle eliminated the self-quenching of BODIPY C12 because of the leakage of BODIPY C12 molecules. The apparent rate constant of the leakage increased with decrease in x OA to 0.60. However, at x OA ≤ 0.60, the apparent rate constants barely changed. The correlation between the leakage of the BODIPY C12 molecules and the transfer of OA molecules enables the evaluation of the lipid transfer during the OA/CHAPSO bicellar mixture formation through the observation of the self-quenching of BODIPY C12.

We evaluated the effect of dilution on both the size and packing density of aggregates prepared from a fatty acid (oleic acid, OA)/detergent (3-[(3-cholamidopropyl)dimethylammonio]-2-hydroxypropane sulfonate (CHAPSO)) bicelle as a parent for functional membrane materials. The sizes of the aggregates formed at different molar ratios, XOA(= [OA]/([OA]+[CHAPSO])), of 0.3 and 0.7 and their parent bicelles were measured by dynamic light scattering and transmission electron microscopy; their packing density was evaluated by deconvolution of the fluorescence spectrum, where Laurdan molecules were used as a probe. The experimental results showed that the bicelles formed aggregates upon dilution because of the hydration of CHAPSO. The packing density of the nano-ordered aggregate formed at XOA = 0.3 was much greater than that of the aggregate formed at XOA = 0.7, implying the formation of an ordered aggregate under the condition of XOA = 0.3