## 5 Selenium-Containing Polysaccharides
### Se-Esters--Selenites and Diselenites of Polysaccharides  
The esterification of polysaccharides with selenium acid results in formation of polysaccharide selenites, esters of selenious acid (Figure 8). Under appropriate conditions, practically all native polysaccharides, as polyhydroxy aldehydes or ketones, may react with H\({}_{2}\)SeO\({}_{3}\) or Na\({}_{2}\)SeO\({}_{3}\) in an acidic environment to yield the polysaccharide selenites. In these compounds, selenium is tetravalent and in the fourth oxidation state, so it retains the redox properties typical for this oxidation state [118]. Depending on the acylation method used and the degree of polysaccharide substitution (DS), the content of selenium in the polysaccharide molecule may be significantly different.  
Currently, the selenylation of natural polysaccharides to selenious acid esters is a frequent method to obtain selenium-containing polysaccharides. The method most commonly used to obtain a selenite from a polysaccharide is using nitric acid and sodium selenite [255].  
In 2009, [PERSON] et al. described the use of this method to introduce selenium into polysaccharides isolated from _Cynomorium songaricum_. The maximum selenic content determined by ICP-AES was 2925 \(\upmu\)g/g [256]. In 2012, [PERSON] et al. reported the synthesis of selenium-containing polysaccharides derivatives from _Artemisia sphaercopla_ polysaccharide with a high Se content (1703 \(\upmu\)g/g) using H\({}_{2}\)SeO\({}_{3}\)/HNO\({}_{3}\) and BaCl\({}_{2}\) as a catalyst [244]. The C-6 substitution was predominant in selenized polysaccharide, as proven by Raman and (13)C NMR spectroscopy. The authors have observed a sharp decrease in the molecular weight of the polysaccharide caused by a degradation of the polymer. The structure of obtained Se-polysaccharides was studied by using size exclusion chromatography combined with multi angle laser light scattering (SEC-MALLS) [257].  
Several modifications of the above-described method for the acylation of natural polysaccharides have been described, using nitric acid and selenious acid [258], glacial acetic acid and selenious acid [259], glacial acetic acid and sodium selenite [258], or selenium oxychloride [259; 260]. Using the above-described methods of selenylation, several research teams obtained biologically active selenites from plant, mushroom, and bacterial polysaccharides in the last decade. The examples include polysaccharides from _Hedysarum polybotrys_[261], _Cordyceps militaris_[262], _Catthelasma ventricosum_[263], _Enteromorpha prolifera_[264], _Lachnum_ sp. [265], _Artractylodes macrocephala_[255], _Lentinual edodes_[266], _Lilum lancifolium_[267], _Codonopsis pilosula_[268], _Lycium barburn_[269], _Astragalus_ sp. [270], _Hericium erinacues_[271], _Glycyrrhiza uralensis_[272], _Epinedium_ sp. [273], _Isatis indigotica_ root [273], _Schisandra chinensis_[243], _Caparis spionosa_[274], _Cynomorium songaricum_[256], _Murus alba_[275], _Castanea mollissima_[276], _Rhizobium_ sp. [277], _Artemisia sphaercopla_[244], _Potentilla anserina_[278], _Lactococcus lactis_ subsp. _lactis_[260], garlic [279], tea [280], and others.  
In addition to the selenylation method used, the selenium content of selenylated polysaccharides (10\({}^{2}\)-10\({}^{4}\)\(\upmu\)g/g) [281] also depends on the structure and chemical character of the native polysaccharide. For example, using the same selenylation method, the ability to incorporate selenium into lily polysaccharides (11,770-39,780 \(\upmu\)g/g) [267] was 26-88-fold higher than that of _Murus alba_ polysaccharides (452 \(\upmu\)g/g) [275].  
Selenium polysaccharides obtained with those methods exhibit a broad spectrum of biological activity, e.g., antioxidant, antibacterial, anti-hyperlipidemic, anti-diabetic, antitumor, hepatotoxicity, neuroprotective, anti-inflammatory, antifibrotic, immunomodulatory, and immunoenhancing activity. This subject will be widely discussed in a separate section (Section 7).