Starch polymorphs

Recent work has, however, revealed the structure of other starch polymorphs not involved in inclusion behaviour of the above type, but which are inclusion compounds in their own right by virtue of their co-crystallisation with water. 

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. 

Veregin, R. et al. (1986) Characterization of the crystalline A and B starch polymorphs and investigation of starch crystalhzation by high-resolution carbon-13 CP/MAS NMR. Macromolecules, 19 (4),... 

In the 1950s, new methods of protein separation were developed that enabled the systematic study of molecular variation in many more human proteins. Starch gel electrophoresis allowed the separation of closely related protein variants by differences in charge and molecular size. Smithies (1955) detected the amazing polymorphism of haptoglobin. In later years the method was extended to the study of allozymes (enzyme polymorphisms). 

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. 

Chain Building. Both crystalline polymorphs of starch have the same fiber repeat of 1.05 nm and are built with the same unit a parallel stranded double-helix. Each strand has six glucose residues per turn in 2.1 nm and the two strands are related by a two-fold axis of symmetry this creates the apparent 1.05 nm fiber repeat. The chirality of the helices has been postulated to be either right-handed (12.13) or left-handed (14.15. ... 

Comparison with the Crystalline Structure of A- and B-Type Starch. In the most recent crystallographic studies on the crystalline part of starch (14.15) the structure of both polymorphs are based on a parallel arrangement of left-handed double-helices. In the two observed structures the double-helices are slightly different since small variations away from the perfect six-fold symmetry are found. Nevertheless, they correspond closely to the model studied here. The essential result is that in these two structures the closest interactions between two neighboring double-helices correspond closely to the duplex described as PARA 1. 

Understanding the Crystalline Polymorphism in Native Starch. From this study, it is clear that the A and B forms have in common not only a double-helix but a... 

Figure 9. Model of the polymorphic transition from the B-type to the A-type starch, in the solid state. The parallel double-helices which form the duplex are labelled 0 and 1/2 (this indicates their relative translation along the c axis). The water molecules are shown as dots.

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. 

Long-range ordering (crystallinity) in starch granules is demonstrated by x-ray diffraction (Chapter 6). WAXD patterns of starches correspond to one of the two limiting polymorphs (A or B) or the combination C form, the C-pattem being a superposition of A- and B-patterns.28 29 The A form is typical of cereal starches, while... 

Figure 8.8 The B- to A-type transition of B-type potato starch lintners (DP 15) at 35% water content (d.b.). The arrows on the DSC thermal curve specify the temperatures at which the lintners were heated prior to x-ray analysis.320 The model of Perez et al.320 for the solid state polymorphic transition B to A is also shown, illustrating the progressive removal ofwater molecules (small dots) and the change in packing of chain duplexes the chain duplexes marked with 0 and Vi indicate their relative translation along the c axis.

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