Structure chitin, solid state

The N-acetyl-D-glucosamine moiety in both chitins can be considered as the magnetic independent residue in full agreement with the crystal structure of a- and p-chitin where this residue is also the crystallographic independent unit. Table 2.12 shows the similarity in a- and p-chitin NMR signals which means that solid-state NMR is not the appropriate technique to differentiate them. 

Chitin is a stable compoimd, incompatible with oxidizing agents [59]. In the solid state imder alkaline condition (e.g., NaOH, KOH, heat at about 120 °C) or by enzymatic hydrolysis in the presence of a chitin deacetylase, it hydrolyses to form the deacetylated degradation product chitosan [6,7,10,11]. It was found that the presence of urea in basic media and at low temperature (—20 °C) had little effect on chitin structure and that urea is of benefit to the stability of chitin solution [38]. 

There are several kinds of natural biodegradable polymers in addition to bacterial PHAs, such as proteins, nucleic acids and polysaccharides. Among them, particulary important polymers such as industrial materials are polysaccharides, such as starch, cellulose, chitin and chitosan. The solid-state structure and properties of starch and amylose [127], cellulose [128] and chitin... 

Also it has been proposed that bonds exist between cuticular protein and chitin (38). Solid state l N-NMR analysis of chitin prepared by alkali extraction of 1, 3-[ 5 ]-histidine labeled M. sexta pupal exuviae revealed an N chemical shift expected for the substituted imidazole nitrogen cross-link structure depicted in Fig. 3 (24). Apparently, the chitin is not coupled directly to protein but, instead, to a diphenolic carbon, which serves as a part of the bridge between protein and chitin macromolecules. 

From among the natural carbohydrate polymers, we mention here cellulose, chitin and its deacetylated form chitosan, hyaluronic acid (hyaluronan), and heparin. Apart from cellulose, the monomer-unit sequences are not strictly regular, but the structures given below are representative. Chitosan, hyaluronic acid, and heparin are water-soluble because they carry electrically-charged functions. Since cellulose and chitin are insoluble in water, most of their radiation chemistry has been done in the solid state, as discussed below. Yields of molecular-weight reduction have usually been determined by viscosimetry and, more recently, by the laser light-scattering technique. 

Van de Velde, K., Kiekens, P. Structure analysis and degree of substitution of chitin, chitosan and dibutyiylchitin by FT-IR spectroscopy and solid state NMR. Caibohydr. Polym. 58, 409 16 (2004)... 

In the solid state, chitin mainly occurs as two allomorphs, the a- and (3-forms, which are differentiated by infrared and solid state NMR spectroscopies and X-ray diffraction [3,4]. a-chitin is the most abundant, occurring in krill, lobsters and crab tendons, shrimp and crab shells. It also results from recrystallization of (3-chitin in solution treated with strong aqueous HCl (over 7N) and washed with water because the a-form is thermodynamically more stable than (3-chitin [5]. (3-chitin is found in association with proteins in squid pens [3, 6] or in the tubes synthesized by pogonophoran and vestimetiferan worms [7,8]. In the solid state, the chains are parallel in (3-chitin and antiparallel in a-chitin. Their crystalline structures were reviewed in different papers [1, 4, 9-13]. Chitin is insoluble in all the usual solvents while taking into account their crystalline forms (a or (3) because the reactivity of p-chitin is larger than that of the a-isomorph, which is important for enzymatic and chemical transformations of chitin [14]. 

CHITIN STRUCTURE IN THE SOLID STATE 8.2.1 General Remarks... 

The solid state NMR is applicable for soluble, insoluble structural analysis of chitin/chitosan samples it can be used for entire range of the DA determination no need to prepare and to dry the sample some information on chemical structure, sequence of co-monomer units and type of copolymers (random and block) can be obtained from the spectra of chitin/ chitosan samples the more sensitive instrument generally results in the higher precision resolution, limit of detection and accuracy of results are improved using cross polarization and strong magnetic fields. 

Most spectroscopic and titration methods have been employed to determine these values and were listed by Rathke (36). The structural differences between chitin polymorphs have been studied by nmr and ir (37,38). Conductometric titrations have been confirmed by solid state nmr to be an accurate but easy method to determine the degree of deacetylation of chitosan (39). 

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