Encapsulation of the Antioxidant R-(+)-α-Lipoic Acid in Permethylated α- and β-Cyclodextrins: Thermal and X-ray Structural Characterization of the 1:1 Inclusion Complexes

Molecular Symmetry of Permethylated β-Cyclodextrins upon Complexation

The Cn molecular symmetry implicated by the schemes with which cyclodextrins (CDs), the well-known cyclic oligosaccharides, are introduced in the literature, is not valid. Numerous studies have shown that CDs are rather flexible with their macrocycle adopting various conformations that enable the inclusion complexation of guest molecules of various shapes. In this work, the loss and gain of the C7 symmetry of the heptakis (2, 3, 6-tri-O-methyl)-β-CD (TM-β-CD) is investigated by means of its conformation geometrical features in its hydrated form and upon complexation with molecules of different shapes. For this, the crystal structure of the inclusion complex of a bulky guest molecule (giberellic acid) in TM-β-CD is presented for the first time and compared with the previously determined crystal structures of monohydrated TM-β-CD and the inclusion complex of a linear monoterpenoid (geraniol) in TM-β-CD. The structural investigation was complemented by molecular dynamics simulations in an explicit solvent, based on the crystallographically determined models. The crucial role of the guest, in the symmetry gain of the host, reveals a pronounced induced-fit complexation mechanism for permethylated CDs.

Cyclohexanohemicucurbit[8]uril Inclusion Complexes With Heterocycles and Selective Extraction of Sulfur Compounds From Water

Solid-phase extraction that utilizes selective macrocyclic receptors can serve as a useful tool for removal of chemical wastes. Hemicucurbiturils are known to form inclusion complexes with suitably sized anions; however, their use in selective binding of non-charged species is still very limited. In this study, we found that cyclohexanohemicucurbit[8]uril encapsulates five- and six-membered sulfur- and oxygen-containing unsubstituted heterocycles, which is investigated by single-crystal X-ray diffraction, NMR spectroscopy, isothermal titration calorimetry, and thermogravimetry. The macrocycle acts as a promising selective sorption material for the extraction of sulfur heterocycles, such as 1,3-dithiolane and α-lipoic acid, from water.

Self-emulsifying drug delivery system of (R)-α-lipoic acid to improve its stability and oral absorption

The present study was designed to develop a self-emulsifying drug delivery system (SEDDS) of (R)-α-lipoic acid (RLA) to improve the physicochemical and nutraceutical properties of RLA. RLA/SEDDS was prepared using medium-chain triglycerides, Tween 80, and polyethylene glycol 400 as oil, surfactant, and co-surfactant, respectively. The preferable composition of SEDDS was selected according to a pseudo-ternary phase diagram for improved emulsification properties, and its physicochemical and pharmacokinetic properties were evaluated. RLA/SEDDS showed the immediate formation of fine micelles with a mean droplet size of approximately 260 nm when introduced into aqueous media. In simulated gastric fluid, this system could significantly improve the dissolution behavior of RLA and prevent the degradation of RLA, possibly due to the encapsulation of RLA into the emulsion structure. Following the oral administration of RLA/SEDDS (10 mg RLA/kg) in rats, systemic exposure to RLA and dihydrolipoic acid (DHLA), a reduced form of RLA, increased by 7- and 3-fold, respectively. The improved dissolution and gastric stability of RLA could contribute to enhancing systemic exposure to RLA and DHLA after oral administration. From these findings, RLA/SEDDS might be an efficacious dosage option for improving the oral bioavailability as well as nutraceutical properties of RLA.

Encapsulation and Stabilization of α-Lipoic Acid in Cyclodextrin Inclusion Complex Electrospun Nanofibers: Antioxidant and Fast-Dissolving α-Lipoic Acid/Cyclodextrin Nanofibrous Webs

In this study, electrospinning of nanofibers from alpha-lipoic acid/cyclodextrin inclusion complex systems was successfully performed without having any polymeric matrix. Alpha-lipoic acid (α-LA) is a natural antioxidant compound which is widely used as a food supplement. However, it has limited water solubility and poor thermal and oxidative stability. Nevertheless, it is possible to enhance its water solubility and thermal stability by inclusion complexation with cyclodextrins. Here, hydroxypropyl-beta-cyclodextrin (HP-β-CyD) and hydroxypropyl-gamma-cyclodextrin (HP-γ-CyD) were chosen as host molecules for forming inclusion complexation with α-LA. Accordingly, α-LA was inclusion complexed with HP-β-CyD and HP-γ-CyD by using very high concentrated aqueous solutions of CyD (200%, w/v) having 1/1 and 2/1 molar ratio of α-LA/CyD. Except α-LA/HP-β-CyD (1/1) solution, other α-LA/CyD solutions were turbid indicating the presence of some noncomplexed α-LA whereas α-LA/HP-β-CyD (1/1) solution was very homogeneous signifying that α-LA was fully complexed with HP-β-CyD. Even so, electrospinning was performed for all of the α-LA/HP-β-CyD (1/1 and 2/1) and α-LA/HP-γ-CyD (1/1 and 2/1) aqueous solutions, and defect-free bead-less and uniform nanofibers were successfully obtained for all of the α-LA/CyD solutions. However, the electrospinning process for α-LA/CyD (1/1) systems was much more efficient than the α-LA/CyD (2/1) systems, and we were able to produce self-standing and flexible nanofibrous webs from α-LA/CyD (1/1) systems. α-LA was efficiently preserved during the electrospinning process of α-LA/CyD (1/1) systems and the resulting electrospun α-LA/HP-β-CyD and α-LA/HP-γ-CyD nanofibers were produced with the molar ratios of ∼1/1 and ∼0.85/1 (α-LA/CyD), respectively. The better encapsulation efficiency of α-LA in α-LA/HP-β-CyD nanofibers was due to higher solubility increase and higher binding strength between α-LA and HP-β-CyD as revealed by the phase solubility test. α-LA was in the amorphous state in α-LA/CyD nanofibers and both α-LA/HP-β-CyD and α-LA/HP-γ-CyD nanofibers were dissolved very quickly in water and also when they wetted with artificial saliva. Additionally, the antioxidant activity of pure α-LA and α-LA/CyD nanofibers was comparatively evaluated using ABTS radical cation assay. α-LA/CyD nanofibers have shown significantly higher antioxidant performance compared to pure α-LA owing to improved water solubility by CyD inclusion complexation. The thermal stability enhancement of α-LA in α-LA/CyD nanofibers was achieved compared to pure α-LA under heat treatment (100 °C for 24 h). These promising results support that antioxidant α-LA/CyD nanofibers may have potential applications as orally fast-dissolving food supplements.