Amylopectin characterization

Com and rice starches have been oxidized and subsequently cyanoethylated (97). As molecular size decreases due to degradation during oxidation, the degree of cyanoethylation increases. The derivatized starch shows pseudoplastic flow in water dispersion at higher levels of cyanoethylation the flow is thixotropic. Com and rice starches have been oxidized and subsequently carboxymethylated (98). Such derivatives are superior in the production of textile sizes. Potato starch has been oxidized with neutral aqueous bromine and fully chemically (99) and physically (100) characterized. Amylose is more sensitive to bromine oxidation than amylopectin and oxidation causes a decrease in both gelatinization temperature range and gelatinization enthalpy. 

Fig. 1. Primary stmctures of some common polysaccharides, (a) Alpha-glycoside linkages characterize amylose, amylopectin, and glycogen (b) cellulose has...

The labile nature of the components necessitates that, for fundamental investigations, the starch should preferably be extracted from its botanical source, in the laboratory, under the mildest possible conditions.26 Industrial samples of unknown origin and treatment should not be used. The characterization of the starch would appear to entail (1) dissolution of the granule without degradation, (2) fractionation without degradation, (3) complete analysis of the finer details of structure of the separated components (including the possibilities of intermediate structures between the extremes of amylose and amylopectin), and (4) the estimation of the size, shape, and molecular-weight distribution of these fractions. 

It is obviously important that the fractionation products should be adequately characterized. The only accurate method for ascertaining the purity of the starch components, and also the amylose/amylopectin ratio in whole starch, is to determine potentiometrically the amount of iodine bound.8 38 Colorimetric methods which have been suggested37 38 are useful for comparative measurements, but are often not absolute. The yield of... 

Further measurements appear necessary before the molecular weights of the amylopectin component of starches can be adequately characterized, and it may well be that light-scattering is the only method which can be satisfactorily applied to these polysaccharides of very high molecular weight. Certainly, it is the only method which enables studies of very dilute solutions to be made with high accuracy, particularly in the case of aqueous solutions. 

Table I lists physical data for a number of the carbamate and ester derivatives of cellulose, chitin, amylose, amylopectin, and dextran synthesized as described in the Experimental Section. The solubility of the polysaccharide starting materials as well as that of the produced derivatives allows for macromolecular characterization through techniques including UV, NMR, IR, high pressure liquid chromatography, etc.

To determine the amylose content of starch, the iodine reaction has been most commonly used because amylose and amylopectin have different abilities to bind iodine. The methods such as blue value (absorbance at 680 nm for starch-iodine complex using amylose and amylopectin standards), and potentiometric and amperometric titration have been used for more than 50 years. These procedures are based on the capacity of amylose to form helical inclusion complexes with iodine, which display a blue color characterized by a maximum absorption wavelength (kmax) above 620 nm. During the titration of starch with iodine solution, the amount (mg) of iodine bound to 100 mg of starch is determined. The value is defined as iodine-binding capacity or iodine affinity (lA). The amylose content is based on the iodine affinity of starch vs. purified linear fraction from the standard 100 mg pure linear amylose fraction has an iodine affinity of 19.5-21.0mg depending on amylose source. Amylopectin binds 0-1.2mg iodine per 100mg (Banks and Greenwood, 1975). The amylose content determined by potentiometric titration is considered an absolute amylose content if the sample is defatted before analysis. 

Fig. 7. Seasonal variations in concentrations and exoenzymatic decomposition rates of organic material in the O to 1 cm horizon of a sandy-mud sediment (water depth 18 m) of the Kiel Bight (Baltic Sea, FRG). Illustrated are concentrations of protein and carbohydrate, and activity ofO(-amylase (mg of amylopectin azure decomposed per g of dry weight sediment per hour). The headline on top characterizes specific ecological situations and events affecting the sediments.

Swinkels29 collected published characterization data for tapioca starch and compared it to that for other starches of commercial significance (Table 12.4). Tapioca starch is differentiated from other starches by its low level of residual materials (fat, protein, ash), lower amylose content than for other amylose-containing starches, and high molecular weights of amylose and amylopectin. The small amount of phosphorus in tapioca starch is partially removable30 and, therefore, not bound as the phosphate ester as in potato starch. It is also common to find protein and lipid values of zero, as reported by Hicks.31 The very low protein and lipid content is an important factor which differentiates tapioca starch from the cereal starches. 

The amylose fraction from three different oat starches was characterized by high-performance size-exclusion chromatography (HP-SEC) and shown to be free of amylopectin.31 The degree of polymerization is given in Table 15.2. By treating amylose with isoamylase, it was found to be branched.31 The major fraction had a DP of 700, whereas a second fraction had a DP of 72. The amylose chain length was reported to decrease with increased amylose and starch-lipid content.30 The intrinsic viscosity of amylose has been reported to be 246-299 mL/g.7... 

Table 15.2 Characterization of amylose, amylopectin and intermediate material in three oat starches31...

Mayer, J. M. (1993). Processability and characterization of starch-blend films produced using different starch sources and amylose amylopectin ratios. 206th National Meeting, Am. Chem. Soc., Div. Food Agric. Sci., Chicago, IL, Abstract 129. 

Since these species were conceptual, there were no well-characterized synthetic examples of such regular architecture. Meyer [73] proposed such a symmetrically, ordered architecture for amylopectin, but subsequent work by Erlander and French [74] and Burchard et al. [75], showed that neither amylopectin nor glycogen is so regular, but are instead, random f-functional polycondensates possessing a large, hyperbranched dextrin core (in Fig. 13). 

Because of the difficulties involved in separating the isoenzymes among themselves and from other enzymes such as amylases, it is not yet clear what the best acceptors are and what the products are for each isoenzyme, but some characterization of the enzymes has been done (e.g., Km for a-glucans such as amylopectin, different animal glycogens, and maltosaccha-rides). Characterization of the products has been minimal, but some progress has been made in our laboratory (see the next section, Mode of Action ). 

These reactions do not have to occur in perfect sequence, and the phases may have some overlap (e.g., phases 2, 3, and 4 may overlap, and possibly even 5 and 6). However, the present evidence, such as intermediate products formed by starch mutants of C. reinhardtii and of higher plants, supports the sequence of reactions shown in Fig. 1 for amylopectin and amylose biosynthesis. Further experiments are required to test this hypothetic scheme, and attempts to purify and characterize the debranching enzyme, crucial to this hypothesis, are under way. 

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