Amyloses molecular shape

Fig. 43. Amylose. The molecular shape of amylose explains some of its chemical and biochemical properties. With iodine, amylose turns intensely blue. This venerable reaction has been explained only recently. It was found that iodine molecules enter the hollow center of the coil created by the glucose units. In such enclosure compounds iodine exhibits changed physical characteristics, such as the strong absorption of light. There are enzymes that slit open this tube and free fragments, each of which contains six glucose units (Section 6).

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. 

Methods which can be used to determine the size and shape of polysaccharides have been reviewed.107 (A critical survey of these has recently been given by Sadron108 and by Ogston.109) Special problems exist in the case of the undegraded starch components. In view of the branched nature of amylopectin and the large size of the amylose molecule, chemical methods of estimating size are inadequate, and it is questionable whether results are valid.38 The free components may also aggregate in aqueous solution. Study of derivatives is therefore more convenient, and the preparation of these is an essential preliminary to estimations of molecular size. 

The I2-starch reaction is the basis of a spectrophotometric assay at 640 nm with amylose standards. A similar Congo red method is less precise than the iodine method, but it has the advantage of insensitivity to the DP, molecular size, and shape. The Congo red assay can therefore be supple-... 

Amylose has a molecular weight in the range 100,000-1,000,000 Daltons, is only lightly branched and in aqueous solutions assumes the shape of a random coil consisting of helical segments joined by segments with no specific conformation. 

The constant a appears to be a function of the shape and solvation of the molecules. Theoretical calculations predict a to be 0.5 for a matted coil, 1.0 for a randomly linked chain, and 2.0 for a stiff chain. Experimental values vary from 0.53 for polystyrene in butanone to 1.5 for amylose in ethylenediamine. For polymers of molecular weight 100,000 to 1,000,000, the equation is further modified to give... 

In its native state, starch is semi-crystalline (about 20-45%) and water insoluble. Native starch granules typically have dimensions ranging from 0.5 to 175 pm and appear in a variety of shapes. It is composed of linear (amylose) and branched (amylopectin) polymers of ot-D-glucose. Amylose has a molecular mass of about 10 -10 g moU while amylopectin has a molecular mass in the range 10 -10 g moU Starch rich in amylose is usually preferred for conversion to TPS as the linearity of amylose improves the processability of starch even though it is present as a minor component (between 20 and 30wt%). The ratio of amylose to amylopectin depends on the source and age of the starch, and can also be influenced by the extraction process. ... 

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