Sugars, as one of the most important carbohydrates (the other two types are starches and fibers), mainly consist of carbon (C), oxygen (O), and hydrogen (H) atoms usually with a hydrogen–oxygen atom ratio of 2:1. Sugars occupy a central position in plant metabolism as they not only are the first complex organic compounds formed in the plant as a result of photosynthesis, but also provide a major source of respiratory energy. And many other classes of plant constituent (e.g. the nucleic acids and the plant glycosides) contain sugars as essential features of their structures. In addition, sugars play a number of ecological roles, in plant—animal interactions (flower nectars are mainly sugar), in protection from wounding and infection and in the detoxification of foreign substances.
The chemical reactions of sugars are largely that of the hydroxyl and carbonyl groups. The aldehyde and ketone groups in sugars undergo redox reactions to produce new substances. The common reactions of sugars include alkylation, acylation, reduction, oxidation, hydrolysis and glycosylation. For the chemical synthesis of complex carbohydrate molecules, the main challenge is to build glycosidic linkages connecting the monomeric units with proper stereo and regiochemical orientation. Here we mainly talk about the glycosylation of sugars.
Glycosylation reaction, as the most important reaction in the field of sugars chemistry, involves the coupling of a glycosyl donor, to a glycosyl acceptor forming a glycoside. The outcome of a glycosylation reaction critically depends on the reactivity of all reaction partners involved: the donor glycoside (the electrophile), the activator (that generally provides the leaving group on the activated donor species) and the glycosyl acceptor (the nucleophile). The reactivity of a glycosyl acceptor is of fundamental importance to the outcome of a glycosylation reaction. Figure 1 illustrates the proposed mechanism of glycosylation reaction in which succinimide serves as the eventual proton acceptor [1, 2].
Fig. 1 An example of glycosylation reaction
Sugar is a versatile and irreplaceable functional ingredient in food. In addition to providing sweetness, sugar is also used to balance acidity, add bulk, prevent spoilage among other functional properties or increase the shelf life of some products. Besides, sugars are the most important regulators that facilitate many physiological processes, such as photosynthesis, seed germination, flowering, senescence, and many more under various abiotic stresses. In recent years, the uses of sugars in medicine have drawn increasing attention.
For example, sugar ligand molecules, such as mannose, galactose and glucose, can bind to drug delivery systems. They have many active hydroxyl groups which can covalently bind with the active groups of drug delivery systems, such as amino and carboxyl groups. These glycosylation ligands have the advantages of nontoxicity, no immunogenicity, good biocompatibility and biodegradation. They can be widely used in glycosylation-modified drug delivery systems.
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