Structural features of a number of xylan types (Sigma, Fluka) and that extracted by us from birch wood (BW) have been determined using IR transmission spectroscopy and elemental analysis. The calculated molecular formulas of Sigma and Fluka xylans are close to the theoretical molecular formula of a xylopyranose residue whereas the molecular formula of BW xylan differs from it. The investigated xylan samples contain both acetyl and methoxyl groups in small amounts. A somewhat higher content of acetyl groups is found in BW xylan compared to that in Sigma and Fluka xylans and it practically does not contain traces of lignin and phenolic alcohols whereas Sigma xylan includes large traces of lignin and glucuronic acids. IR spectra of the investigated samples suggest that the studied types of xylan have an identical system of hydrogen bonds.

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Use of nitrate oxidants (HNO 3 or N 2 O 4 ), each of which causes both oxidation and hydrolysis of cellulose, is proposed for one-or two-stage transformation of native cellulose into microcrystalline cellulose topochemically modified with carboxyl groups. Conversion of the acid form of the preparations into the salt forms (sodium, lithium) makes it easy to obtain stable thixotropic hydrogels, revealed to be effective enterosorbents and drug vehicles in clinical trials.The products of oxidative-hydrolytic modification of cellulose are of interest for medicine not only as effective hemostatic and bactericidal agents, but also as enterosorbents. The widely distributed carbon and aluminum-containing enterosorbents cannot completely replace enterosorbents of polysaccharide nature food fibres, which have adapted to the human gastrointestinal system over the entire course of evolution. The traditional use of natural polyuronyls (pectins, polysaccharides from marine algae) in enterosorption is complicated by the possible accumulation of toxic substances from the biosphere in the vegetation period. Microcrystalline cellulose (MCC) merits special attention from the point of view of the high degree of chemical purity of polysaccharides not digestible in the human body [1]. During its preparation, extraneous contaminants primarily accumulated in the structurally unordered constituents of the fibrous precursor are removed together with this constituent during hydrolytic splitting of the fibre to the so-called limiting degree of polymerization. In addition, MCC has the unique ability to form stable thixotropic hydrogels [1], which significantly expands the possibilities of using it in the pharmaceutical and food industries. However, the efficacy of MCC as an enterosorbent is limited by the absence of the ion-exchange groups contained in natural polyuronides (pectins, laminarin, carrageenan).Fabrication of material that combines ion-exchange properties with the properties of MCC, primarily the gel-forming power, is probably only possible in conditions of intercrystallite interaction of the cellulose with the oxidant, where oxidation of crystallites is limited to their surface. This situation predetermines the relatively high degree of oxidation of cellulose, which excludes the reaction from turning into an intracrystallite reaction [2]. We propose two variants of conversion of native cellulose into carboxylated microcrystalline cellulose (CMCC) using one reagent capable of executing both oxidative and hydrolytic functions from the beginning.In one variant, the air-dried cellulose was saturated with nitrogen(IV) oxide vapors in a specially manufactured reactor and held at room temperature for the time required to attain the necessary degree of oxidation. Then a calculated amount of water was fed into the reactor and the contents were heated to the boiling point. Hydrolytic degradation of the polysaccharide, catalyzed by the mixture of nitric and nitrous acids formed, takes place in these conditions. Their concentration in terms...

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Features of a non-traditional method of preparation of cellulose-based cation-exchange enterosorbents using nitrogen (IV) oxide are discussed. The proposed approach involves sequential use of oxidative and acid-hydrolyzing functions of nitrogen (IV) oxide to convert ordinary cellulose into carboxylated microcrystalline cellulose. X-ray phase analysis and transmission electron microscopy data show evidence of a topochemical character of cellulose microcrystal modification with carboxyl groups. Factors are indicated causing enhanced stability and whiteness of the samples thus obtained as compared with oxidized cellulose fibers, as well as the ease of stable tixotropic gel formation from the samples prepared in salt form. Exemplified by methylene blue sorption, a comparative assessment of the sorptive ability of enterosorbent powders and hydrogels prepared in acid and salt forms is made. Results of clinical trials conducted with the gel enterosorbents in the areas of gastroenterology and detoxication therapy are presented. In view of the compatibility of carboxylated microcrystalline cellulose in hydrogel form with many biologically active ingredients, prospective character of using the enterosorbents not only as such but also as a matrix for the development of new dosage forms of other medications is pointed out. Examples showing advantages of such combinations in specific sustained release drug forms (ointments and microgranules) are given.