Polysaccharides utilisable

Analytical techniques that utilise biopolymers, ie, natural macromolecules such as proteias, nucleic acids, and polysaccharides that compose living substances, represent a rapidly expanding field. The number of appHcations is large and thus uses hereia are limited to chiral chromatography, immunology, and biosensors. 

Coupling reagents of the dialkylcarbodiimide type are most frequently utilised for the esterification of polysaccharides with complex carboxylic acids [179]. The best known condensation agent of this class, particularly in peptide and protein chemistry, is N.N -dicyclohexylcarbodiimide (DCC, Fig. 24) [187]. 

Although CDI was applied as early as 1972 as reagent for the esterification of starch and dextran, it has only scarcely been used up to now. Its renaissance during the last few years may be due to the fact that it became an affordable commercially available product. Among the first attempts for the esterification of polysaccharides via CDI is the binding of amino acids onto dextran. Besides CDI, -(thiocarbony diimidazole can be utilised to obtain the corresponding imidazolide [198]. The amino acids bound via this path are glycine, L-leucine, L-phenylalanine, L-histidine and L-alanyl-L-histidine. They are protected with N-trifluoroacetyl, N-benzyloxycarbonyl... 

Interestingly, CDI can also be utilised for the introduction of substituents by inter- or intramolecular coupling of OH moieties of the polysaccharide via a carbonate function. This synthesis was used to obtain dextran with 2-hydroxyethyl methacrylate moieties (dex-HEMA) and dex-HEMA with lactate spacer functions (Fig. 32). A new class of dextran derivatives (DS < 0.2) that can be polymerised containing hydrolysable groups is accessible [177]. 

Carbohydrates would be the predominant raw materials for future biorefineries. The major polysaccharides found in nature are cellulose, hemicellulose and starch (see Chapter 1). These molecules would be mainly utilised after they are broken down to their respective monomers via enzymatic hydrolysis, thermochemical degradation or a combination of these two. Cellulose and hemicellulose, together with lignin, constitute the main structural components of biomass. Starch is the major constituent of cereal crops. This section would focus on the potential utilisation of carbohydrates and lignocellulosic biomass for chemical production. 

There is now a great deal of evidence that liposaccharide intermediates are involved in some biosyntheses of polysaccharides in animals and bacteria. This is an alternative to the UDP-sugar process (11.60) and utilises various nucleoside diphosphate sugars together with a polyprenol phosphate. The presence of a long hydrocarbon chain in the latter is believed to facilitate biosynthesis in the hydrophobic regions of membranes. 

Many plants (especially those growing in semi-arid conditions) produce gummy exudates when the bark is damaged which serve to heal the wound. The exudate often dries to a hard amorphous mass, and is produced in sufficient abundance by some species of trees and shrubs to warrant collection and commercial utilisation. Gum exudates are readily obtainable in relatively pure, undegraded form though in some cases purification is still required. These polysaccharides often occur in association with a protein. 

Mass spectrometric methods are routinely used to characterise a wide variety of biopolymers, such as proteins, polysaccharides, and nucleic acids. Nevertheless, despite its advantages, MS has been under utilised in the past for studying synthetic polymer systems. It is fair to say that, until recently, polymer scientists have been rather unfamiliar with the advances made in the field of MS. 

Bacteria isolated from marine slime films, possess a wide range of proteolytic and polysaccharide-degrading hydrolytic enzymes. Vibrio parahaemolyticus absorbs specifically to chitin particles and plankton present in the marine film. The organisms possess enzymes capable of degrading and utilising the chitin components of the film. 

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