Synthesis of Cyclodextrin

Many studies have been conducted on the synthetic route of cyclodextrin (CD) since it has a wide range of applications. As enzymatically modified starch, CD is most commonly synthesized by the enzymatic breakdown of polymers where D-glucopyranose units are linked by α-1,4-glycosidic bonds, such as starch or amylose, which is carried out by cyclodextrin glycosyltransferase (CGTase).

In addition, CD can also be obtained via templated enzymatic synthesis, which corresponds to the reverse process of the enzymatic breakdown of polymers. In this method, CGTase catalyzes reversible transglycosylation to construct CD molecules from small building blocks such as maltose.

Enzymatic Breakdown

During enzymatic breakdown, starch such as amylopectin and amylose are used as substrates and undergo different transglycosylation steps catalyzed by CGTase from different sources, which can produce non-reducing cyclic dextrins, and then undergo intramolecular cyclization to form CD.

The "Solvent Processes" is a type of enzymatic production process that is selective for the production of CD due to the involvement of a complexing agent. In this process, the organic complexing agent selectively precipitates one type of CD, thus directing the enzymatic reaction to produce mainly that type of CD, which is favorable for use in the industrial-scale production of most CDs. A typical flow diagram of the "Solvent Processes" is shown in Fig. 1 ^[1].

• Liquefaction of starch

The first step is liquefaction of starch, which is carried out using thermostable amylase, acid, mechanical disintegration or thermostable CGTases. On an industrial scale, liquefaction is usually achieved by jet-cooking.

• Enzymatic conversion

After the starch is liquefied, the starch solution is cooled to the enzyme reaction temperature, and CGTase and organic complexing agent are added. Enzymatic conversion in the presence of CGTase then occurs to produce CD, after which the desired CD selectively forms complexes with the complexing agent and precipitates.

• Downstream processing

The complexing agents are separated and obtained by centrifugation or filtration of the CD-reagent complexes from the reaction solution, followed by washing, and steam distillation or liquid-liquid extraction. And the product solution is concentrated by reduced pressure distillation, activated carbon treatment, followed by crystallization, filtration and drying to obtain the desired CD product.

Fig. 1 "Solvent Processes" for CD production ^[1].

Enzymatic Synthesis

In addition to the commercial production of CDs via the enzymatic breakdown of starch, CDs can also be synthesized directly from substrates via CGTase-catalyzed reversible glycosylation in the presence of templates. The process of making CD from disaccharide building blocks is shown in Fig. 2 (G1 represents glucose, G2 represents maltose, and the black rectangle represents the leaving group). There are two steps involved in this enzymatic synthesis process ^[2].

• Cleavage of substrate: The α-1,4-glycosidic bond in the substrate is cleaved by CGTase to form a covalent enzyme-substrate glycosyl donor complex.
• Construction of CD: A glycosyl acceptor replaces the enzyme in a covalent enzyme-substrate complex and forms a new glycosidic bond. If this process is an intramolecular reaction, specific CD molecule can be constructed.

Fig. 2 CGTase catalyzes the cleavage of specific units in maltose (G2) to build specific
CDs, with glucose (G1) as a byproduct ^[2].

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