Linearly Branched Polysaccharides

Linearly branched polysaccharides, i.e. polymers with a long backbone chain and with many short side chains, such as guaran or alkyl cellulose, have properties which are a combination of those of perfectly linear and of branched molecules. The long backbone chain is responsible for high solution viscosity. The presence of numerous short side chains greatly weakens interactions between the molecules, as shown by the good solubility and rehydration rates of the molecules and by the stability even of highly concentrated solutions. 

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Normal com starch is composed of 20—30% of the linear polysaccharide amylose and 70—80% of the branched polysaccharide amylopectin. 

However, fine grinding of iasoluble dietary fiber such as bran reduces WHC. In general, branched polysaccharides are more soluble than are linear polysaccharides because close packing of molecular chains is precluded. WHC is strongly kifluenced by the pentosan components of cell-waU dietary fiber and varies with the stmcture and source of these hemiceUuloses. 

Molecular Interactions. Various polysaccharides readily associate with other substances, including bile acids and cholesterol, proteins, small organic molecules, inorganic salts, and ions. Anionic polysaccharides form salts and chelate complexes with cations some neutral polysaccharides form complexes with inorganic salts and some interactions are stmcture specific. Starch amylose and the linear branches of amylopectin form inclusion complexes with several classes of polar molecules, including fatty acids, glycerides, alcohols, esters, ketones, and iodine/iodide. The absorbed molecule occupies the cavity of the amylose helix, which has the capacity to expand somewhat to accommodate larger molecules. The starch—Hpid complex is important in food systems. Whether similar inclusion complexes can form with any of the dietary fiber components is not known. 

Recent progress of basic and application studies in chitin chemistry was reviewed by Kurita (2001) with emphasis on the controlled modification reactions for the preparation of chitin derivatives. The reactions discussed include hydrolysis of main chain, deacetylation, acylation, M-phthaloylation, tosylation, alkylation, Schiff base formation, reductive alkylation, 0-carboxymethylation, N-carboxyalkylation, silylation, and graft copolymerization. For conducting modification reactions in a facile and controlled manner, some soluble chitin derivatives are convenient. Among soluble precursors, N-phthaloyl chitosan is particularly useful and made possible a series of regioselective and quantitative substitutions that was otherwise difficult. One of the important achievements based on this organosoluble precursor is the synthesis of nonnatural branched polysaccharides that have sugar branches at a specific site of the linear chitin or chitosan backbone [89]. 

Yu, L.P. and Rollings, J. E., Quantitative branching of linear and branched polysaccharide mixtures by size exclusion chromatography and on-line low-angle laser light-scattering detection, /. Appl. Polym. Sci., 35, 1085, 1988. 

Figure 14.2 Representative oligosaccharide structures found on mammalian glycoproteins and glycolipids. The complex oligosaccharides may be bi-, tri-, or tetra-antennary the branches may be more or less elongated with 1—>4 linked lactosamine units, and they may or may not be sialylated. The SLex, Lea, and Leb structures represent the different blood group determinants often present on lipids, and the elongated core 2 structure is a mucin-type glycosylation. Proteoglycans have a common core to which a variety of linear acidic polysaccharides are attached.

In the case of branched polysaccharides, incorporation of side chains may take place through different mechanisms. In one of them, the assembly of the main chain is independent of the presence of side chains, and their incorporation into a polymeric molecule occurs as a modification of an initially formed, linear polysaccharide. Another situation is possible when incorporation of monosaccharide residues present in side chains is a necessary condition for elongation of the main chain, either through the monomeric or the block mechanism that is, intermediate formation of a linear, polysaccharide chain does not occur. Both mechanisms of incorporation of side chains were demonstrated to take place. 

Abbreviations for biosynthetic types are composed from abbreviations of nucleoside residues of activated forms of the monosaccharide components. 6 See footnote a to Table VI lin. and br. mean linear and branched polysaccharides. c Including other bacterial amphi-philes of Gram-negative, outer membranes. d Including amphiphiles of Gram-positive cell-membranes. 

Polysaccharides are polymers of monosaccharides. Starch is a polysaccharide composed of amylose, an essentially linear polysaccharide, and amylopectin, a highly branched polysaccharide both are polymers of D-Glucose. 

The phase separation threshold is lower for systems containing a branched polysaccharide than for systems containing a linear polysaccharide of the same molecular weight. It is higher for globular proteins compared to proteins of unfolded structure. An increase in excluded volume means a decrease in the free volume of the solution accessible for biopolymers. Thus, the excluded volume of biopolymer molecules implies that water in real foods can be nonsolvent water relative to macromolecules. 

Starch, cellulose, and murein are three of the most abundant organic compounds found on earth. Starch is of particular interest as it is the starting material for production of IMO. Starch exists as a mixture of amylose and amylopectin. Amylose, is a linear polysaccharide comprised of D-glucose units linked a-( 1 4) to each other and amylopectin is a branched polysaccharide composed of D-glucose units linked a-(1 4), with 5% of linkages a-(l— 6) [72]. The starch component of many plants is a mixture of amylose and amylopectin, although the starches from waxy varieties of maize, rice, sorghum, and barley are completely composed of amylopectin. 

For carbohydrates to meet these requirements, diversity is needed on both the molecular and the size-level. Only large carbohydrate molecules, polysaccharides, can provide the wide spectrum of storage, structural, and gel-forming abilities required by nature. Meeting these requirements has made it necessary for plants to produce polysaccharides that can be classified as linear, branched, and crosslinked polymers, as well as homo- and heteropolymers in accordance with terminology in common use in the polymer community (O Fig. 1) [26]. Nature has found need to adopt all different kinds of macromolecular architectures in pursuit of the three different functions of carbohydrates. 

The characteristic components of the cytoplasmic membrane of Gram-positive bacteria are the amphiphilic macromolecules lipoglycan and lipoteichoic acid [39,40] (O Fig. 8). Lipotei-choic acids possess in their saccharide chain alditol phosphates as characteristic components, whereas lipoglycans are linear or branched polysaccharides linked to diacylglycerol. These molecules may be further substituted by phosphoglycerol residues. Since lipoteichoic acids and lipoglycans are not found in the same bacterium they are believed to replace each other... 

For the synthesis of glycogen-type polysaccharides from a-n-glucopyran-osyl phosphate, two enzymes are required. Phosphorylases, in presence of a suitable primer, synthesize linear chains of -( —> 4)-linked n-glucose residues these are then converted into a branched polysaccharide by a branching enzyme. ... 

The specificity of potato Q-enzyme h been examined in some detail. With amylose as donor substrate, the DP must exceed 40 n-glucose residues for rapid transglucosylation to occur with a branched polysaccharide as donor, the outer chains must exceed 14 n-glucose residues. Since linear chains of this length are not attacked by Q-enzyme, there are evidently multiple sites for the binding of Q-enz3une and its substrate. 

Isoamylase branched polysaccharides [endoacting a-1,6] linear polysaccharides Pseudomonas amyloderamosa Flavobacterium odorratum... 

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