Alkali chitin

In concentrated NaOH, chitin becomes alkali chitin which reacts with 2-chloroethanol to yield 0-(2-hydroxyethyl) chitin, known as glycol chitin this compoimd was probably the first derivative to find practical use (as the recommended substrate for lysozyme). Alkali chitin with sodium monochloroacetate yields the widely used water-soluble 0-carboxymethyl chitin sodium salt [118]. The latter is also particularly susceptible to lysozyme, and its oUgomers are degraded by N-acetylglucosaminidase, thus it is convenient for medical appHcations, including bone regeneration. 

Typical derivatives of chitin (1) and chitosan (2) [67] alkali-chitin (3), O-acylchitin (4), O-carboxymethylchitin (5), O-hydroxyethylchitin (6), chitin O-sulphate (7), chitin O-phosphate (8), chitosan salt (9), /V-acylchitosan (10), /V-carboxymethylchitosan (11), trialkylammonium salt (12)... 

As stated above, the present biomedical products utilize only the base materials. Chanical derivatization is the way forward to realize the fuU potential of chitin and chitosan (Alves and Mano 2008). The well-known C6-earboxymethyl-chitin and ehitosan would be an ideal starting place (Raimunda and Campana-Filho 2008). However, this work again started with the alkali-chitin process that is known to be ineffieient and associated with inhomogeneity eoncems. Another recent work is the preparation of Af-hydroxyacryl-chitosan that utilizes ehitosan from a commercial source without further treatment (Maa et al. 2008). Phosphorylated chitins and ehitosans are another example of chemical derivatization that yields soluble compounds (Jayakumar et al. 2008). 

Chitin-coated cellulose (produced by treating cellulose with alkali-chitin) has been used in the purification of lysozyme and a derivative of lysozyme by affinity chromatography. DNA derivatives of cellulose have been used in the chromatography of phosphorylated, nucleolar nonhistone and in the purification of a glucocorticoid receptor from rat liver. 

Figure 1. Some chemical reactions for the molecular design of chitin and chitosan. [1], chitin [2], chitosan [3], alkoxide (alkali chitin) [4] salt (carboxyl-ate) [5], chelation [6] Schiff s base [7], N-acylation [8], halogenation [9], N-alkylation [10], 0-alkylation [11], oxido-deaminative cleavage [12], 0-acylation [13], sulfonation [14], sulfation, phosphorylation and nitration.

In aqueous cold alkali, chitin also dissolves and behaves as a macromolecular solution, as verified by its mechanical spectrum when tested by small-deformation oscillatory measurements. At 4°C and 1.5% (w/w) the system could be described as a transparent sol phase with a viscoelastic response characteristic of an entangled concentrated network, which is comparable to the spectrum exhibited by chitosan (in 0.1 mol acetic acid), but no yield stress is... 

Nevertheless, if a gradual increase in temperature is applied, the alkali-chitin solution undergoes phase separation, yielding a dilute (solvent-rich) sol phase and a concentrated (solute-rich) gel phase (see Fig. 2). 

Fig. 2. Alkali chitin in aqueous solution at 20°C and after phase separation-gelation upon heating to 70°C...

In Fig. 3 are shown the results obtained with a 1% alkali chitin solution. The cloud point can be determined as the temperature at which a very slight increment in the absorbance arises. From optical measurements it could be appreciated that the cloud point is centred at 30 C (Fig. 3a). The appearance of a slight opalescence in the solution at this temperature is in very close correspondence with the onset of changes in q equilibrium values. The latter are manifested by a well defined break-point in the Arrhenius-type plots (Fig. 3c). The dependence of the break-point values of each ti (T ) plot with frequency, CD, is also evident. The behaviour of the system was also monitored by the ratio of fluorescence intensities of pyrene at 384 and 372 nm, when excited at 343 nm. From the curve depicted in Fig. 3b, it can be appreciated a decrease in the fluorescence ratio when the system is heated with an inflexion point at 21.5°C (as determined by the first derivative method), signalling the onset of self-aggregation of the polymer. 

The cloud point values thus obtained for alkali chitin solutions in the range 0.15 to 1.5%, clearly describe lower limiting phase separation behaviour, as can be observed from Fig. 4. The lower critical solution temperature (LCST) is apparently located at T = 30°C (1% concentration), where there is a minimum in the cloud point curve. It should be noted the excellent agreement between the three experimental techniques employed at this polymer concentration. 

Both, natural and synthetic polymers with associative properties arising from hydrophobic interactions give aqueous solutions with LCST. Among the most known systems having LCST behaviour should be mentioned polyethylene glycol-water and aqueous solutions of methyl cellulose. Also, in poly(methacrylic) acid, LCST phase diagrams were determined from the change in shear modulus and turbidity. For alkali chitin, the main key role played by hydrophobic interactions in LCST is evident from the decrease in the fluorescence ratio observed in Fig. 3b. 

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