Vietnamese chitin raw material, the chitin de- N -acetylation reaction, and a new chitosanalginate gelling concept

Abstract

Chitin is a linear biopolymer composed of 2-acetamido-2-deoxy-D-glucopyranose (N-acetylglucosamine or GlcNAc, A-unit) linked by ß – (1-4) glycosidic linkages. Chitin occurs as a structural polysaccharide in animals with an outer skeleton (Arthropoda), and in the cell wall of certain fungi. In the cuticle of crustaceans and insects, chitin exists in close association with proteins, minerals and pigments. In Vietnam, a country endowed with favorable conditions for aquaculture, the annual shrimp production from aquaculture is approximately 450 000 metric tons (2010), and one third of this is byproducts, including head and shell. The two major species are white shrimp (Penaeus vannamei) and black tiger shrimp (Penaeus monodon). These shrimp by-products are a large resource not only for chitin but also for other valuable components as proteins and pigments.

The chemical composition of heads and shells of the black tiger and the white shrimp was analysed. The amounts of the three main components, i.e. proteins, chitin, and minerals, were found to be similar in the by-products from the two shrimp species. The protein contents of the heads were 44.39 ± 0.50 % and 48.56 ± 1.33 % of the dry weight in the white shrimp and black tiger shrimp, respectively, which were about 50% higher than in the shells. In the shells, the chitin content were 27.37 ± 1.82 % and 29.29 ± 1.78% of the dry weight in the white shrimp and black tiger shrimp, respectively, which were more than 2.5 times higher than in the heads. These large differences in the chemical composition of the heads and the shells had consequences for the optimal extraction conditions in order to isolate a pure and high molecular weight chitin from isolated heads and shells. The amino acid composition of the proteins were similar for the two species, both for heads and shells, and with a profile that was suitable as a source for fish feed.

Chitin is insoluble in aqueous solvents, which limits its applications. However, by partly removing chitin’s acetyl groups and thereby introducing amino groups that can be protonated and positively charged (D-units), the water-soluble polysaccharide chitosan can be prepared. This is performed by chemical de-N-acetylation of chitin at highly alkaline conditions and high temperature. The de-N-acetylation reaction was studied in detail with the chitin disaccharide (GlcNAc-GlcNAc or AA) as a model substrate. The resonances in 1H NMR spectrum of the chitin disaccharide in 2.77 M NaOD were assigned. The ß-anomeric protons of the four different disaccharides, i.e. AA, DD, AD, and DA, are well separated and can be monitored during the de-N-acetylation. Thus, the rate of de-N-acetylation of the reducing end was found to be twice the rate of the nonreducing ends. The total rate of de-N-acetylation of chitin disaccharide was for the first time determined to be second order with respect to sodium hydroxide concentration. This contributes to explain the differences between the homogeneous and heterogeneous de-N-acetylation reaction. The activation energy for the reaction was determined to 114.4 and 98.6 kJ/mol in 2.77 M and 5.5 M NaOD, respectively.

Hydrogels of biopolymers have attracted much attention for their applications in e.g. tissue engineering, immobilization of cells and controlled drug release. A new gelling system of chitosan – alginate, or their corresponding oligomers, is described. The gelling system was studied by combining either poly-mannuronate and chitosan oligomers, or polymeric chitosan and mannuronate oligomers. The two components were mixed at a pH well above the pKa-values of the amino-groups, where the chitosan/chitosan oligomers are almost uncharged, allowing mixing with the negatively charged poly-mannuronate/mannuronate oligomers without the precipitation that would otherwise occur upon mixing a polyanion with a polycation. Then the pH was lowered by adding D-glucono-δ-lactone (GDL), a proton donating substance with the ability to release protons in a controlled way, so that the amino groups of chitosan/chitosan oligomers were protonated and thereby positively charged, resulting in the formation of a hydrogel. The neutral-solubility of the polymeric chitosan is achieved by selecting a polymeric chitosan with a degree of acetylation of 40%, while the neutral-solubility of the (fully de-N-acetylated) chitosan oligomers is obtained by selecting oligomers with a chain length below 10. The kinetics of gelation was fast in both gelling systems, with a sol-gel transition within the time for the first measurements. Initial rates of gelation and gel strengths (measured as storage modulus, G’) increased with increasing concentration of oligomers. The gel strength (G’) of both gelling systems increased with increasing GDL concentration (and thereby the final pH of the gel) from neutral pH down to pH 4, and decreased with increasing ionic strength, indicating that ionic hydrogels are formed.

The importance of the nearly perfect match in distance between the negative charges on the same side of poly-mannuronate/mannuronate oligomers and the positive charges on the same side of chitosan/chitosan oligomers is crucial for these gelling systems, as demonstrated by the very different gel strengths of two alginates with extreme composition, i.e. a poly-mannuronate and a poly-guluronate, where poly-mannuronate formed relatively strong gels with chitosan oligomers while poly-guluronate formed gels of very limited mechanical strength.