Glucosamine units

In spite of the alteration due to deacetylation, chitosan from crab tendon possesses a crystal structure showing an orthorhombic unit cell with dimensions a = 0.828, b = 0.862 and c = 1.043 nm (fiber axis). The unit cell comprises four glucosamine units two chains pass through the unit cell with an antiparallel packing arrangement. The main hydrogen bonds are 03 05 (intramolecular) and N2 06 (intermolecular) [82]. This material has also found medical uses (below). 

Degradation of heparin followed by affinity chromatography on immobilized AT III led to the identification of a pentasaccharide unit with high antithrombin affinity. This pentasaccharide 3 is characterized by a imique highly sulfated central glucosamine unit with a 3-0-sulfate group (Structures 2). 

FIGURE 7-18 Chitin. (a) A short segment of chitin, a homopolymer of N-acetyl-o-glucosamine units in 01 -e>4) linkage, (b) A spotted June beetle (Pellidnota punetatia), showing its surface armor (exoskeleton) of chitin. 

Chitin is a long-chain polymer (similar to cellulose) composed of /S-N-acetyl glucosamine unit (Meyer and Mark [53]). Every unit contains two OH groups and one NH-COCH3 group ... 

In the more abundant a chitin the chains in alternate sheets have opposite orientations,101102 possibly a result of hairpin folds in the strands. Native chitin exists as microfibrils of 7.25 nm diameter. These contain a 2.8-nm core consisting of 15-30 chitin chains surrounded by a sheath of 27-kDa protein subunits. The microfibrils pack in a hexagonal array, but the structure is not completely regular. Several proteins are present some of the glucosamine units of the polysaccharide are not acetylated and the chitin core is often calcified.103 The commercial product chitosan is a product of alkaline deacetylation of chitin but it also occurs naturally in some fungi.102 Chitin is also present in cell walls of yeasts and other fungi. It is covalently bonded to a P-l,3-linked glycan which may, in turn, be linked to a mannoprotein (see Section D,2)97... 

Meyer H.Wehrli, Helv 20, 353(1937) CA 31, 5807(1937) 5)M.L.Wolfrom et al, JACS 65, 2084(1943) 6)M.L.Wolfrom et al, Ohio State Univ Final Rept, Project 459, Columbus, Ohio(Jan 1953)pp 1,3 34 Nitrated Deacetylated Chitine A wh flocculent ppt contg 11=6 to 11,9% nitrate N(as detd by DuPont nitrometer obtd by Wolfrom et al(Ref 3) by nitrating deacetylated chitin with 100% HNOj. The nitrated product represented the combined nitrate salt nitrate ester of deacetylated chitin. This substance contd one nitric acid salt unit per anhydro-o-glucosamine unit and 1.60-1.65 nitrate ester unit per anhydro-o-glucosamine anhydro -N-acetyl-D -glucosammine units. Attempts to nitrate deacetylated chitin by means of nitrogen pent oxide, in a non-aqueous medium y in the presence of sodium fluoride, by the method of Caesar(Ref 2) were unsuccessful mainly because of occlusion of NaF in the nitrated products(Ref 3)... 

The chemistry and biological activities of lipid A are reviewed in earlier chapters of this volume. As noted there, the components of the lipid A complexes from Salmonella species and j5. coli are derivatives of a disaccharide comprised of two D-glucosamine units linked B,l->-6. The disaccharide is substituted at positions 1 and 4 by phosphate functions and on the amino nitrogens by B-hydroxymyristoyl groups. A variable number (up to 5) of ester-linked fatty acyl residues is also present. In the simplest components the phosphates are present as monoester groups. 

Chitosan and its derivatives are the most widely used cationic polymeric excipients. Chitosan consists of (31—>4 D-glucosamine units and is derived by the deacetylation of chitin from insects, crustaceans and fungi. It interacts ionically with the anionic substructures of sialic acid residues on the mucus layer. Chitosans are rapidly hydrated in a low pH environment like the gastric fluid and do not swell above pH levels of 6.5, exhibiting no more mucoadhesion. 

Chitin A natural polysaccharide obtained from crab shells The chain is based on acetylated glucosamine units the natural polymer can be converted to the acid-soluble partially deacetylated product chitosan... 

Once it was realized that the most important group of the neuraminic acid is the carboxylate, this saccharide was frequently replaced by a, in some cases substituted, hydroxyacetic acid group. The glucosamine unit was substituted by a simple glycol as in 90 [233] or a substituted glycol, e. g., the C2-symmetrical 2,3- butanediol group [234,235], also C-glycosidic mimetics... 

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