Amylose linear

Amylose (linear starch) contains only a-l,4-glycosidic linkages involving the groups indicated by the arrows... 

One of the industrially available natural polymers for the manufacture of biodegradable plastics materials is starch. Starch is produced and stored by various types of plants. The differing properties of various starches result from their different proportions of amylose (linear starch chains) and amylopectin (branched starch chains). 

Starch Amylose linear chain Native starch high... 

Amylose linear, a-1,4 glycosidic links. MW 4,000-150,000. Deep blue color with iodine due to coiled complex enclosing iodine-colour is lost with heating, returns when cooled. 

Comparison of projection drawings and calculated chain dimensions for amylose, linear "dextran", and pullulan chains shows that the a-1,6-linkage is a source of considerable conformation freedon in D-glucan chains v ich incorporate this linkage. Experimental studies of the coil dimensions of pullulan and other polysaccharides containing a-1,6-linkages will be required to refine the parameters of the theoretical model for this linkage. 

Properties of starch are influenced by the ratio of amylose (linear chains of glucose molecules) to amylopectin (branched chains of glucose molecules). Common starches contain 17 to 27% amylose, and 73 to 83% amylopectin. Waxy starches contain nearly 100% amylopectin. These do not form gels except at high concentrations they remain as relatively clear solutions. Waxy starches can be derived from corn, grain sorghum, and rice. 

Starch is made up of two polymers of glucose units, amylose (linear chain) and amy-lopectin (branched polymer). It is used as the basis to manufacture talcum powder and powder make-up products due to its soothing and emollient properties. It is possible to modify its original structure to obtain products that produce gels with a variety of properties, which widens its practical applications (dextrin, pre-gelatinized starch, sorbitol, introduction of functional gronps). 

Starch gelatinisation process (a) raw starch granules made up of amylose (linear) and amylopectin (branched) (b) addition of water breaks up crystallinity and disrupts helices (c)) addition of heat and more water causes swelling, amylose diffuses out of the granule (d) granules, mostly containing amylopectin, are collapsed and held in a matrix of amylose (adapted from Lai and Kokini, 1991). 

The most important natural binder still in use today is starch, though it is now frequently used in combination with synthetic binders. Com starch is more common in the USA, whereas potato starch is more prevalent in Europe. Native starch containing two fractions of amylose (linear chain) and amylopectin (branched chain) is not suitable for coating paper and board because the amylose fraction tends to undergo retrogradation and the viscosity of coating colors made with native starch is too high [5]. For these reasons, only treated (i.e., depolymerized) or chemically modified starches are used. Most paper mills carry out their own starch preparations in-house. 

Cellulose and amylose are comprised of the same glucose subunits as the cyclodexttins. In the case of cellulose, the glucose units are attached through 1,4-P-linkages resulting ia a linear polymer. In the case of amylose, the 1,4-a-linkages, as are found ia the cyclodexttins, are thought to confer heUcity to the polymeric chain. 

The distinctions between these homopolymers arise from the different ways in which the monomer units are hooked together in polyacetal chains. Starch (qv), plant nutrient material, is composed of two polysaccharides a-amylose and amylopectin. cx-Amylose is linear because of exclusive a (1 — 4) linkages, whereas amylopectin is branched because of the presence of a (1 — 6) as well as a (1 — 4) links. The terms linear and branched refer only to primary stmcture. 

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

An aqueous dispersion of an unmodified starch containing amylose wiU gradually form an insoluble precipitate through association of linear segments. This process is called retrogradation or set-back. 

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. 

The small change in stereochemistry between cellulose and amylose creates a large difference in their overall shape and in their properties. Some of this difference can be seen in the strorcture of a short portion of fflnylose in Figure 25.9. The presence of the a-glycosidic linkages imparts a twist to the fflnylose chain. Where the main chain is roughly linear- in cellulose, it is helical in anylose. Attractive forces between chains are weaker in fflnylose, and fflnylose does not form the same kind of strong fibers that cellulose does. 

FIGURE 7.21 Amylose and amylopectin are the two forms of starch. Note that the linear linkages are o (1 4), but the branches in amylopectin are o (1 6). Branches in polysaccharides can involve any of the hydroxyl groups on the monosaccharide components. Amylopectin is a highly branched structure, with branches occurring every 12 to 30 residues. 

Extensive studies have been performed on the (1- 6)-)8-D-glucan (pustulan) and the (l- 4)-a-D-glucan (amylose). These are linear polysaccharides that may exist as helical polymers in aqueous solution, as demonstrated by c.d. spectroscopy. Characteristic of the helical structure of these glucans is a negative band at 182 nm, a crossover at 177 nm, and a more intensely positive band at shorter wavelengths (see Figs. 8 and 9). 

The most abundant organic molecule in the biosphere is cellulose, a polysaccharide that is the principal building material for plants. Like amylose, cellulose is a linear polymer of glucose. Unlike amylose, however, the glucose monomers in cellulose are in the ji configuration (see Figure IS-lSt. 

Figure 17-5. Amylose, cellulose. Amylose consists of a water-soluble portion, a linear polymer of glucose, the amylose and a water-insoluble portion, the amylopectin. The difference between amylose and cellulose is the way in which the glucose units are linked. In amylose, a-linkages are present, whereas in cellulose, p-linkages are present. Because of this difference, amylose is soluble in water and cellulose is not. Chemical modification allows cellulose to become water soluble.

Starch is the most important polysaccharide in the human diet. It consists of two t)q)es of molecules fhe linear and helical amylose and fhe branched amylopectin. Depending on fhe source, sfarch generally contains 20-25% amylose and 75-80% amylopectin. In contrast to the two polysaccharides mentioned in the Section III.C, it is a major energy... 

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