1. Cyclodextrins: general considerations

Due to its structure, cyclodextrins (CDs) readily form inclusion complexes through noncovalent interactions with molecular guests. The lipophilic cavity of CDs provides a microenvironment into which appropriately sized nonpolar moieties can enter. The hydrophobicity of the cavity enables the accommodation of a broad range of hydrophobic guests such as the alkyl chains of surfactants or different phytochemicals [1]. The hydrophilic exterior usually imparts CDs and their complexes, considerable solubility in water. The charge and polarity of the guest molecule play also an important role in the CD-substrate host-guest interaction [2]. However, this aspect is obviously less important than the geometric fitting. In the case of the charge, the complexation of neutral molecules is easier than the ionized counterpart. In general, molecules can be encapsulated by CDs when they are less hydrophilic or less polar than the solvent and when the formed complex is stable.

compounds present in nature (i.e., more than 8000 different structures have been identified up to now) results from variations in the basic chemical skeleton (e.g., degree of oxidation, hydroxylation, methylation, glycosylation, and conjugation with further molecules, particularly lipids, proteins, other phenolics, and biomolecular metabolites) [9]. Phenolic compounds are grouped by the number of phenol rings they contain and the structural elements that bind these rings to another; flavonoids, phenolic acids, tannins, stilbenes, and lignans are examples

Interactions between Bio-Based Compounds and Cyclodextrins

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On the other hand, plant essential oils (EOs) are mixtures of numerous highly complex volatile compounds (hydrogenated and oxygenated monoterpenes, sesquiterpenes, phenols, simple alcohols, ketones, coumarins, etc.) present in variable concentrations whose aroma depends on the individual constituents present [11]. Due to their natural properties, EOs have been used as therapeutic remedies and flavoring agents since ancient times. In the last decades, many investigations showed that EOs have a wide range of valuable biological activities, such

Although many investigations demonstrated the broad range of biological activities of many phytochemicals, they still have restricted applicability as pharmaceuticals or in food products due to their poor water insolubility and bioavailability, high volatility, rapid oxidation, or degradation when exposed to environmental factors. New approaches have been developed to overcome these drawbacks, and among them, CDs have been suggested as excellent vehi-

According to a recent review by Suvarna et al., there are many phytochemicals whose solubility, bioavailability, or therapeutic activity is significantly improved by complexation with CDs (e.g., quercetin, curcumin, artemisinin, resveratrol, naringenin, etc.) [12]. The methods used for the formation of inclusion complexes between CDs and bioactive compounds are essentially

The encapsulation of phytochemicals with CDs usually involves the formation of 1:1 inclusion complexes with the most versatile CD, the β-CDs, and its derivatives. These derivatives can be classified according to their interaction with the water molecules in hydrophilic, hydrophobic, and ionizable derivatives [13]. Examples of used hydrophilic β-CDs are the methylated β-CDs—2,6-dimethyl-β-CD (DM-β-CD) and 2,3,6-trimethyl-β-CD (TM-β-CD) the hydroxyalkylated β-CDs such as 2-hydroxypropyl-β-CD (HP-β-CD), and the branched β-CDs, glycosyl-β-CD (G-β-CD). These molecules are suitable for the formation of host-guest inclusion complexes with poor water-soluble compounds. On the other hand, the hydrophobic derivatives, such as the alkylated β-CD 2,6-diethyl-β-CD (DE-β-CD), are used to decrease and modulate the released rate of water-soluble molecules. Finally, the ionizable β-CD can enhance the dissolution rate and the inclusion capacity and even decrease the side effects of some molecules [16, 17]. Among the ionizable CDs, O-carboxymethyl-β-CD (CM-β-CD), O-carboxymethyl-O-ethyl-β-CD (CME-β-CD), and sulfate and sulfobutylether-β-CD (SBE-

cles for the protection of phytochemicals for food and drug delivery proposes [12–14].

neutralization, slurry, solution, coprecipitation, kneading, and grinding [15].

of important representatives of these groups [10].

as antimicrobial, herbicidal, insecticidal, antioxidant, etc.

2.1. Interaction between phytochemicals and cyclodextrins

β-CD) should be highlighted.

The main driving force for the formation of the complex is the release of enthalpy-rich water molecules from the cavity; water molecules are displaced by more hydrophobic guest molecules present in the solution to achieve the apolar-apolar interactions and decrease of CD ring strain resulting in a favorable lower energy state. The beneficial modification of guest molecular properties after the formation of the inclusion complex leads to a large number of applications in areas as diverse as encapsulation of active substances (i.e., flavoring agents, metallic cations, fragrances, and pesticides), enzymatic synthesis, catalysis, and energy transfer studies [3, 4]. Additionally, CDs also find important uses in cosmetics, environment protection, bioconversion, packing, textiles, and food domain [1, 5].

Less than 10% of all produced CDs and CD derivatives are used by the pharmaceutical industry. The largest CD users are the food and the cosmetic industry. CDs have a high level of biocompatibility, are absorbed in the gastrointestinal tract, and are completely metabolized by the colon microflora [6]. Some of them are approved by the Food and Drug Administration or have been accredited as being "generally recognized as safe" (GRAS) [7]. In the cosmetic area, CD performance stands out in the following: solubilize and stabilize specific sensitive components, stabilize emulsions, improve the absorption of active components onto the skin, reduce or eliminate bad aromas from certain components, and reduce the loss of the active components through volatilization, rapid oxidation, destruction by light, etc. [8].
