Supramolecular Chemistry of Cyclodextrin - Cyclodextrin / Alfa Chemistry

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Supramolecular Chemistry of Cyclodextrin

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Supramolecular Chemistry of Cyclodextrin

Supramolecular chemistry is an emerging field in chemistry that deals with constructing complex chemical systems from a discrete number of molecules. It is an integral tool in the design and development of functional systems. The concepts proposed by supramolecular chemistry include host-guest chemistry, molecular self-assembly, molecular folding, and molecular recognition. Among them, molecular recognition and self-assembly of molecules are salient features of supramolecular systems.

In supramolecular chemistry, cyclodextrin (CD) is one of the most important compounds, which acts as a host to form a host–guest inclusion complex with guests and constructs a supramolecular system. Molecular recognition and self-assembly of CD are very important research topics in supramolecular chemistry.

Molecular Recognition

Lehn proposed that "molecular recognition, transformation and translocation represent the basic functions of supramolecular species." ^[1], where molecular recognition refers to the process in which a guest selectively bonds to a host. The inclusion process of CD essentially refers to the process in which guests and CDs recognize each other through intermolecular interactions, and ultimately the guests are partially or completely embedded inside the CD cavities. Therefore, CD can form host-guest inclusion complexes with many organic, inorganic or biological molecules (Fig. 1), which rely on its molecular recognition function.

Fig. 1 Schematic illustration of the formation of an inclusion complex between a CD (host) and a guest ^[2].

The recognition of a variety of small molecules by CD and its derivatives is the most studied field in the supramolecular chemistry of CD. The molecular recognition of native CDs and CDs with charge centers is mainly introduced here.

Native CDs

Different CD cavities recognize or encapsulate different molecules. The cavities of native CDs have a certain degree of rigidity, so the matching degree with the size of the guest molecules directly affects the stability of the resulting host-guest inclusion complexes. Generally, the cavity size of α-CD is suitable for inclusion of monocyclic aromatic hydrocarbons, and the cavity size of β-CD matches the size of a naphthalene ring, as well as γ-CD is suitable for inclusion of tricyclic aromatic hydrocarbons, such as anthracene and phenanthrene ^[3].

CDs with charge centers

Substituents on chemically modified CDs with charge centers can produce electrostatic interactions with certain guest molecules, which is a complement to van der Waals forces and hydrophobic interactions in intermolecular interactions. Occasionally, electrostatic interaction may be the primary driving force behind the formation of CD inclusion complexes, endowing CDs with unique molecular recognition functions.

Self-Assembly

Self-assembly is defined as a process in which molecules or other assembly units spontaneously form ordered aggregates through weak interactions. CD molecules can be connected by covalent or non-covalent bonds, which is the basis for building a supramolecular system. Through non-covalent interactions between molecules and recognition between bonding sites, multiple CD molecules in aqueous solution can self-assemble to form complex and ordered molecular aggregates or supramolecular systems with specific functions. This system can further encapsulate guest molecules for better applications.

Advantages

The self-assembly of CD exhibits the following advantages:

Various structural forms

By self-assembling CD molecules, a supramolecular system is formed that is capable of precisely controlling its molecular structure at a molecular level, giving the supramolecular polymer unique structural characteristics and physicochemical properties that are completely different from the original monomers. The structural forms of the supramolecular system are diverse, including superlattices, micelles, reverse micelles, and vesicles. For example, β-CD aggregates may be an amorphous, micelle-like compound due to the strong π- stacking during the self-assembly process ^[4].

Better inclusion properties

The self-assembly function of CD confers better inclusion properties on CD in addition to its ability to be used to design the desired structures. A self-assembled CD aggregate has a higher binding constant and higher loading capacity for guests than single CD monomers, resulting in a lower limitation on guest-host inclusion complexation ^[4].

References

Lehn, J.-M. Supramolecular Chemistry—Scope and Perspectives Molecules, Supermolecules, and Molecular Devices (Nobel Lecture). Angew Chem Int Ed Engl. 1988, 27(1): 89-112.
Kfoury, M.; et al. Characterization of Cyclodextrin/Volatile Inclusion Complexes: A Review. Molecules 2018, 23(5): 1204.
Saenger, W.; et al. β-Cyclodextrin dodecahydrate: crowding of water molecules within a hydrophobic cavity. Angew Chem Int Ed Engl. 1978, 17(9): 694.
Juan, H.; et al. Research progress on the inclusion mechanism of cyclodextrin-based supramolecular self-assemblies. Food Science. 2023, 44(3): 258-268.

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