Inclusion complex formation of cyclodextrin with its guest and their applications

Abstract

Cyclodextrin (CD) are cyclic oligosaccharides consisting of glucopyranosyl units linked by α-(1,4) bonds. The widely used natural cyclodextrins are α-, β-and γ-cyclodextrin consisting of 6, 7 and 8 glucopyranose units, respectively. The cyclodextrin molecules have a unique structure with a hydrophobic cavity and a hydrophilic surface which can form inclusion complex with a wide variety of guests. The use of cyclodextrins and their derivatives for the encapsulation of bioactive compounds can protect the compounds from environmental conditions and improve the aqueous solubility for increasing their capacity to functionalize the products. In some cases, there is a need to enhance water solubility of β-cyclodextrin by adding the hydroxyalkyl groups on the β-cyclodextrin surface. This review aimed to summarize the method for inclusion complex formation of cyclodextrin with its guests and its applications. Cyclodextrin Cyclodextrins (CD) (or cycloamyloses, cyclomaltoses and Schardinger dextrins) are cyclic oligosaccharides consisting of glucopyranosyl units linked by α-(1,4) bonds [1]. The widely used natural cyclodextrins are α-, β-and γ-cyclodextrin consisting of 6, 7 and 8 glucopyranose units, respectively. The main properties of those cyclodextrins are given in Table 1. The cyclodextrin molecules have a unique structure with a hydrophobic cavity and a hydrophilic surface in which a guest molecule can be entrapped. Cyclodextrin can form inclusion complex with a wide variety of solid, liquid and gaseous compounds. Complex formation is a dimensional, geometrically limited fit, between cyclodextrin cavity and the guest molecule [2]. Generally, hydrophobic molecules have greater affinity for the cyclodextrin cavity when they are in water solution. Moreover, the encapsulation changes the physical and chemical properties of the guests, such as solubility and stability. Therefore, cyclodextrins are suitable for application in food and flavors, pharmaceutical products, cosmetic, agriculture and chemical industries. Cyclodextrins can link specifically to other cyclodextrins (covalent or noncovalent). Because of this property, cyclodextrins can be used as building blocks for the construction of supramolecular complexes. Building blocks which cannot be prepared by common methods can be applied, such as separation of complex mixtures of molecules and enantiomers [3]. Cyclodextrins are produced by reacting gelatinized starch with the enzyme cyclodextrin glucosyltransferase (CGTase) as a result of intramolecular transglycosylation reaction from degradation of starch. Inclusion complex formation Cyclodextrins have an internal non-polar hole and hydroxyl groups placed on the surface, the inclusion of hydrophobic compounds takes place mainly by hydrophobic interactions between guest molecules and the walls of cyclodextrin cavity [5]. However, other forces, such as van der Walls and dipole-dipole interactions, may be involved in the binding of the guest. Despite the number of factors and different forces involved in the complexation with cyclodextrins, the production of complexes is a rather simple process. There are several methods to obtain cyclodextrin-guest complexes depending on the properties of the guest and the nature of the chosen cyclodextrin. Kneading method Kneading technique is suitable for poorly water-soluble guests, because the guest is dissolved slowly during the formation of complex [6]. It affords a very good yield of inclusion formation but it is unsuitable for large scale preparation [7]. Firstly, the liquid or dissolved solid guest is added to slurry of cyclodextrin and kneaded (in a mortar), and then the paste is dried. The obtained solid is washed with a small amount of solvent to remove the free particles adsorbed on the cyclodextrin surface and then dried under vacuum. The inclusion complex formation of cyclodextrins by kneading method has been reported in the encapsulation of ibuprofen [8], omega-3 fatty acids in thymol essential oil [9], thyme essential oil [9] and European anchovy (Engraulis encrasicolus L.) oil [10].