Polymorphs amylose

And as one final observation, the structures of the native amylose polymorphs suggest once more that synthetic polymer chemists have much to learn from natural polymer processes. At this time, only nature seems to be able to assemble such an elegant supermolecular structure as the starch granule, perfectly suited to its needs. 

Bias, B., Le Bail, P., Robert, P. et oL (2(X)6) Structural and stoichiometric studies of complexes between aroma compound and amylose. Polymorphic transitions and quantification in amorphous and crystalhne areas. Carbohydrate Polymers, 66, 306-315. 

FIGURE 5.8 Unit cells (outlined in each diagram) and helix packing in A and B polymorphs of starch. Reprinted from Carbohydrate Research, Vol. 61, Wu and Sarko (1978b), The double helical molecular structure of crystalline A-amylose, Pages 27-40, with permission from Elsevier. 

In the present work, we extend the method to compensate for the hydrogen bonds present in carbohydrates. The hydroxylated character of carbohydrate polymers influences between-chain interactions through networks of hydrogen bonds that occur during crystallization. Frequently, several possible attractive interactions exist that lead to different packing arrangements, and several allomorphic crystalline forms have been observed for polysaccharides such as cellulose, chitin, mannan and amylose. The situation is even more complex when water or other guest molecules are present in the crystalline domains. Another complication is that polysaccharide polymorphism includes different helix shapes as well. 

Progress in the i.r. and Raman spectroscopy of polysaccharides was reviewed by Blackwell.194 In this review, work on polymorphic forms of amylose, on oriented films of glycosaminoglycans, and on bacterial polysaccharides was summarized. The usefulness of the vibrational spectra in determining the conformations of polysaccharides was shown. 

Since its introduction several years ago, the virtual bond, constrained optimization method has proved very useful in studies of polysaccharide crystal structure. Notable among the successes that can be ascribed to it are the structural determinations of the double-helical amylose (.11), the cellulose polymorphs of different chain polarities (.12, 13), and of a number of other polysaccharides and their derivatives. As described in a review of amylose structures elsewhere in this volume, the use of this refinement method has produced structural detail that has previously been unavailable (ll). These results have provided much-needed... 

Unit cell dimensions of different polymorphs of amylose and amylose derivatives. All unit cells contain 2 chains except Va-BuOH and A- and B-amyloses, which contain 1+ chains. 

The double-helical structures of native A- and B-amyloses are found in the fourth group. It is interesting that in both h as well as the d and dyg spacings, they are comparable with the structure of amylose triacetate I (ATAI). In part, this may arise because the packing of the bulky acetate substituents in ATAI is similar to the close-packing of two amylose chains into a double helix. In the latter, one chain may act as the "substituent" for the other chain. At any rate, all three structures contain similar, cylindrical-shaped helices. Somewhat unexpectedly, the distances cL and d-yo are very close for the two native polymorphs, even though their unit cells and packing are... 

Figure 8.17 (a) Melting profiles of the amylose-glycerol monostearate complex (polymorph /) as a... 

The capacity of starch to stain blue-black with iodine suggests that some of the amylose is present in the starch in the V-form. The lipids present in cereal starch would bind to amylose if it were in the V-form, and yet X-ray analysis does not show the presence of the V-polymorph in cereal starches (i.e., most of the amylose would be in the amorphous form). The conclusion is that although a significant part of the amylose is probably in the helical form, the three-dimensional order necessary to give a crystalline diffraction pattern is absent. Indeed, the crystalline nature of starch is now attributed to the presence of amylopectin and not to amylose. Starch from waxy mutants contains only amylopectin (and no amylose), but this starch has the same degree of crystallinity and the same X-ray pattern as the regular starches that contain both components. 

The solid nature of the excipient may influence the final physical form of the tablet (Byrn et al. 2001), such as a tendency to stick (Schmid et al. 2000), or may induce a polymorphic conversion of the active ingredient (Kitamura et al. 1994). Hence, there have been attempts to develop protocols for the selection of compatible active ingredient-excipient compositions (Serajuddin et al. 1999). For instance, nuclear magnetic resonance spectroscopy has been employed to study the structural changes in epichlorohydrin cross-linked high amylose starch excipient (Shiftan et al. 2000), and has also been used to discriminate between two polymorphs of prednisolone present in tablets with excipients, even at low concentrations (5 per cent w/w) of the active ingredient (Saindon et al. 1993). The characterization of excipients by thermal methods has also been reviewed by Giron (1997). 

Shiftan, D., Ravenelle, F., Mateescu, M. A. and Marchessault, R. H. (2000). Change in the V/B polymorph ratio and T-1 relaxation of epichlorohydrin crosslinked high amylose starch excipient. Starch-Starke, 52, 186-95. [243]... 

Conversion of B- to A-amylose on the molecular level occurs with the loss of significant amounts of water followed by a movement of amylose chains into the lattice site vacated by the columnar water of hydration. Starch polymorphism in plants may be a result of the environment in which synthesis occurs. Synthesis and subsequent crystallization may occur as follows amylose single strands are synthesized first, the strands then intertwine about each other forming the amylose double helix. Crystallization then occurs in either the A or B polymorphic form depending on the amount of water in the environment. This mechanism probably implies low degree of crystallinity in the final material, which is generally the case. 

Starch shows distinct crystalline structures that, depending on the source, can easily be identified by X-ray diffraction. The three polymorphisms that can be identified are label A, B and C [11,22]. The most commonly observed structures in native starch are A and B, the former being associated mainly with cereal starches, while the latter generally dominates in tuber starches but also occurs in maize starches with more than 30-40% amylose [11]. Structure... 

In other instances, it is the solvent of crystallization which plays a decisive role in orienting the crystallization toward one or the other polymorph. Such solvent dependant crystallization is well documented in the case of amylose (7) where minute changes in solvent/precipitant ratio have a dramatic effe ct on shifting the recrystallized amylose among three polymorps amylose A,B and V. 

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