Conversion of Chitin to Defined Chitosan Oligomers: Current Status and Future Prospects

Abstract

Chitin is an abundant polysaccharide primarily produced as an industrial waste stream during the processing of crustaceans. Despite the limited applications of chitin, there is interest from the medical, agrochemical, food and cosmetic industries because it can be converted into chitosan and partially acetylated chitosan oligomers (COS). These molecules have various useful properties, including antimicrobial and anti-inflammatory activities. The chemical production of COS is environmentally hazardous and it is difficult to control the degree of polymerization and acetylation. These issues can be addressed by using specific enzymes, particularly chitinases, chitosanases and chitin deacetylases, which yield better-defined chitosan and COS mixtures. In this review, we summarize recent chemical and enzymatic approaches for the production of chitosan and COS. We also discuss a design-of-experiments approach for process optimization that could help to enhance enzymatic processes in terms of product yield and product characteristics. This may allow the production of novel COS structures with unique functional properties to further expand the applications of these diverse bioactive molecules.

Keywords: chitin; chitosan; chitosan oligomers; deacetylation; depolymerization; design of experiments; enzymatic conversion.

Figure 1

Chemical structures of cellulose, chitin and its fully deacetylated derivative chitosan. Whereas cellulose is a polymer of glucose, N-acetylglucosamine (GlcNAc) and glucosamine (GlcN) are the monomeric constituents of the chitinous polysaccharides.

Figure 2

Applications of chitin, chitosan and chitosan oligomers (COS) ordered according to their application fields, market volume and product quality. Biomedical applications are split into two segments: applications for bulk chitosan and applications for more defined COS [14,25,31].

Figure 3

Chemical and biological methods for the extraction of chitin and its sequential conversion to chitosan and chitosan oligomers.

Figure 4

Overview of different pathways of the enzymatic deacetylation of chitin oligomers. (A): Deacetylation of a chitin tetramer by M. rouxii CDA in a ‘multiple attack mode’. (B): A chitin tetramer that is deacetylated by a C. lindemuthianum CDA. This is done in a ‘multiple chain mode’. (C): Subside capping deacetylation model used to explain the generation of partially deacetylated COS using a recombinant CDA from Puccinia graminis f. sp. Tritici. R: sugar with reducing end; grey box: GlcNAc; white box: GlcN; %: relative conversion yield; halved box: three different variants can be generated from the precursor oligomer as only one of the acetylated sugars gets deacetylated. [94,99].

Figure 5

Comparison of a factorial design experiment (a) and the one-factor-at-a-time (OFAT) approach (b) for the maximization of the product yield by optimizing two representative process factors: pH and temperature. a: The factorial experimental design allows the investigation of all possible combinations of the two factors on different levels considering potential interdependencies between the factors within the design-space. Optimum conditions for both factors that give maximum yield can be determined. b: In the OFAT approach, both factors are investigated sequentially (1st, 2nd) neglecting factor interactions, leading to the loss of conclusive information and thereby missing the true maximum.