Chitin is the second most abundant natural biopolymer after cellulose. The chemical structure of chitin is similar to that of cellulose with 2-acetamido-2-deoxy-β- d -glucose (NAG) monomers attached via β(14) linkages. Chitosan is the deacetylated (to varying degrees) form of chitin, which, unlike chitin, is soluble in acidic solutions. Application of chitinous products in foods and pharmaceuticals as well as processing aids has received considerable attention in recent years as exotic synthetic compounds are losing their appeal. This review summarizes some of the important developments related to food applications of chitin, chitosan and their derivatives.

Food applications of chitin and chitosans

Organic chemists have always been fascinated by the cyclopropane subunit.1 The smallest cycloalkane is found as a basic structural element in a wide range of naturally occurring compounds.2 Moreover, many cyclopropane-containing unnatural products have been prepared to test the bonding features of this class of highly strained cycloalkanes3 and to study enzyme mechanism or inhibition.4 Cyclopropanes have also been used as versatile synthetic intermediates in the synthesis of more functionalized cycloalkanes5,6 and acyclic compounds.7 In recent years, most of the synthetic efforts have focused on the enantioselective synthesis of cyclopropanes.8 This has remained a challenge ever since it was found that the members of the pyrethroid class of compounds were effective insecticides.9 New and more efficient methods for the preparation of these entities in enantiomerically pure form are still evolving, and this review will focus mainly on the new methods that have appeared in the literature since 1989. It will elaborate on only three types of stereoselective cyclopropanation reactions from olefins: the halomethylmetal-mediated cyclopropanation reactions (eq 1), the transition metal-catalyzed decomposition of diazo compounds (eq 2), and the nucleophilic addition-ring closure sequence (eqs 3 and 4). These three processes will be examined in the context of diastereoand enantiocontrol. In the last section of the review, other methods commonly used to make chiral, nonracemic cyclopropanes will be briefly outlined.

Stereoselective Cyclopropanation Reactions

Use of natural biopolymers for diversified applications in life sciences has several advantages, such as availability from replenishable agricultural or marine food resources, biocompatibility, biodegradability, therefore leading to ecological safety and the possibility of preparing a variety of chemically or enzymatically modified derivatives for specific end uses. Polysaccharides, as a class of natural macromolecules, have the tendency to be extremely bioactive, and are generally derived from agricultural feedstock or crustacean shell wastes. Cellulose, starch, pectin, etc. are the biopolymers derived from the former while chitin and chitosan are obtained from the latter. In terms of availability, chitin is next to cellulose, available to the extent of over 10 gigatons annually. The application potential of chitosan, a deacetylated derivative of chitin, is multidimensional, such as in food and nutrition, biotechnology, material science, drugs and pharmaceuticals, agriculture and environmental protection, and recently in gene therapy too. The net cationicity as well as the presence of multiple reactive functional groups in the molecule make chitosan a sought-after biomolecule. The latter offers scope for manipulation for preparing a broad spectrum of derivatives for specific end use applications in diversified areas. The biomedical and therapeutic significance of chitin/chitosan derivatives is a subject of significant concern to many all over the world. An attempt is made in this overview to consolidate some of the recent findings on the biorelated application potential of chitosan and its derivatives.

Chitin/chitosan: modifications and their unlimited application potential—an overview

Of the truly abundant polysaccharides in Nature, only chitin has yet to find utilization in large quantity. Chitin is the second most abundant natural biopolymer derived from exoskeletons of crustaceans and also from cell walls of fungi and insects. Chitin is a linear beta 1,4-linked polymer of N-acetyl-D-glucosamine (GlcNAc), whereas chitosan, a copolymer of GlcNAc (approximately 20%) and glucosamine (GlcN, 80%) residues, is a product derived from de-N-acetylation of chitin in the presence of hot alkali. Chitosan is, in fact, a collective name representing a family of de-N-acetylated chitins deacetylated to different degrees. Both chitin/chitosan and their modified derivatives find extensive applications in medicine, agriculture, food, and non-food industries as well. They have emerged as a new class of physiological materials of highly sophisticated functions. Their application versatility is a great challenge to the scientific community and to industry. All these are the result of their versatile biological activity, excellent biocompatibility, and complete biodegradability in combination with low toxicity. Commercial availability of high-purity forms of chitin/chitosan and the continuous appearance of new types of chitin/chitosan derivatives with more and more useful and specific properties have led to an unlimited R&D efforts on this most versatile amino polysaccharide, chitin to find new applications, which are necessary to realize its full potential. Incidentally, this too has become an environmental priority. No doubt, chitin is surely an undisputed biomolecule of great potential.

Chitin--the undisputed biomolecule of great potential

Chitosan, a polycationic polymer and waste product from the sea food processing industry, is an abundant natural resource that has, as yet, not been fully utilized. Advantages of this polymer include availability, low cost, high biocompatibility, biodegradability and ease of chemical modification. In this paper, the physicochemical properties of chitosan, as well as its numerous applications, are reviewed with particular emphasis on its use in water treat ment, pharmaceutics, agriculture and membrane formation.

Applications and Properties of Chitosan

Mise au point d'un materiel immobilise a activite enzymatique conservee, utilisable pour la production industrielle de penicilline semi-synthetique. L'immobilisation de penicilline acylase sur un support type chitosane est decrite: differentes formes du support sont testees: poudre, particules ou billes de chitosane. Differentes methodes d'immobilisation sont egalement presentees: adsorption-reticulation, liaison covalente. Deux types de reacteur sont utilises suivant le type de support choisi: reacteur agite pour la poudre et les particules de chitosane, reacteur lit fixe pour les billes. L'influence de ces differents types d'immobilisation sur l'activite enzymatique de la penicilline G acylase est mesuree

https://onlinelibrary.wiley.com/doi/pdf/10.1002/bit.260330217

The immobilization of penicillin G acylase on chitosan.

Influence of polymerization of the d-amino-acid oxidase on the behaviour of the enzyme covalently immobilized on chitosan.



d-Amino-acid oxidase is a flavoprotein using FAD as cofactor. The enzyme has been immobilized in the presence of FAD on a non-porous matrix: chitosan. This support is covalently bound to the enzyme with glutaraldehyde as cross-linking reagent. It is characterized by a good mechanical resistance to mechanical stirring. The enzymatic assays have been performed in batch reactor with d-phenylglycine as substrate by a spectrophotometric method which is based on the variation of the absorbance at 252 or 280 nm.



The behaviour of the biocatalysts has been studied during repeated assays of 1 h at 25 °C in the absence of exogeneous FAD. The experimental results have been compared with those obtained with the soluble enzyme tested in the presence or in the absence of FAD. The dependence of d-amino-acid oxidase on FAD concentration has been studied. Immobilized enzyme on chitosan appears to be less sensitive to the association-dissociation equilibrium of FAD. This property and the capacity of the enzyme to polymerize spontaneously in solution according to the experimental conditions have been established. The fact that the enzyme can exist in various oligomeric forms is of major importance because its catalytic expression is dependent of this phenomenon. The polymerization is known to be responsible for a decrease of the maximal rate V of the enzyme. It has also been shown that in the same way this decrease was accompanied by an improvement of the affinity of enzyme for substrates. Furthermore, the value of the dissociation constant of the apoenzyme-FAD complex is significantly smaller as the degree of polymerization is high. The conclusion is that the dissociation of the cofactor can be avoided if the immobilization step is carried out at high concentration of enzyme which is favourable to its polymerization.

https://febs.onlinelibrary.wiley.com/doi/pdf/10.1111/j.1432-1033.1988.tb14078.x

J. Braun

Papers

The immobilization of penicillin G acylase on chitosan.

Influence de la polymérisation de la d‐aminoacide oxidase sur le comportement de l'enzyme immobilisée sur chitosane par fixation covalente

An Enantioselective Chemoenzymatic Synthesis of the Four 1-Amino 2-Methylcyclopropane Carboxylic Acids

Resolution of Solketal by Enzymatic Hydrolysis of Its Decanoyl Ester

A Practical Synthesis of (R)- and (S)-3-Hydroxyglutaric Acid Monoesters by Enzymatic Hydrolysis with a Bacterial Esterase.