The three-dimensional structure of A-amylose crystals, as a model of the crystal domains of A-starch granules, was revised using synchrotron radiation microdiffraction data collected from individual micron-sized single crystals. The resulting datasets allowed a determination of the structure with conventional X-ray structure determination techniques normally used for small molecules and not for polymers. Whereas the gross features of this improved structure do not differ extensively from previous structure determination, the high resolution of the diffraction diagrams, which is unusual for a crystalline polymer, allowed the resolution of important new fine details. These include a distortion of the amylose double helices resulting from the occurrence of two intracrystalline molecules of water and a tight network of hydrogen bonds involving each of the primary and secondary hydroxyl groups of the glucosyl moieties. Pairs of water molecules are located in discrete pockets that do not interfere with one another. In addition, the refinement of the new structure indicates a "parallel-down" organization of the amylose molecules within the unit cell as opposed to the previous "parallel-up" model. This new feature indicates that within the crystals, the nonreducing ends of the amylose molecules are oriented toward the c-axis direction of the unit cell. The description of this geometry is important to correlate the crystallography of the granules of A-starch with their ultrastructure and their mode of biosynthesis.Here, we present for the first time the resolution of the structure of a polymer crystal from a full X-ray dataset collected on micron-sized polymer single crystals using synchrotron radiation microdiffraction. This achievement is a substantial advance, which opens the way to many more studies since the technique of growing polymer and biopolymer single crystals is well established.3

The structure of short dough and biscuit has been characterized at a macroscopic level (dimensions, bulk structure) and a microscopic level (starch damage, protein aggregates, microstructure) by physical and biochemical methods. The baking process of short dough induces a large decrease of the product bulk density from 1.26 to 0. 42 (+/-0.01) g.cm(-)(3) for final biscuit, leading to a cellular solid with a thin colored surface and a porous inner structure. Proteins appear aggregated in biscuit when compared to short dough, whereas starch granules remain almost intact in biscuits. The components which are involved in the cohesiveness of short dough and biscuit final structure have been identified. They suggest that short dough is a suspension of solid particles in a liquid phase being an emulsion of lipids in a concentrated sugar solution. The role of sugars in biscuit structure suggest that biscuit structure is a composite matrix of protein aggregates, lipids and sugars, embedding starch granules.

The retrogradation, or reprecipitation, of dilute amylose and amylopectin aqueous solutions was investigated by transmission electron microscopy (TEM). Negative staining, shadowing, and cryo-TEM were combined to study the morphology of the molecular assemblies at different stages of precipitation. Amylose fractal-like networks formed within a few days. They are described as clusters of elementary semicrystalline 10-15 nm units, formed by associations of molecules into parallel double helices, linked by amorphous sections containing loosely organized chains. These networks subsequently condensed, yielding thick aggregates. Amylopectin was found to form similar networks whose branches also had a 10-15 nm lateral width. The elementary units are thought to be clusters of nanocrystals formed by association of the short side branches of the molecule into parallel double helices. As the amylopectin networks were stable in solution at this concentration during several months and did not undergo any further aggregation, the branched configuration of the molecule is believed to hinder the long-scale rearrangement of the crystallites.

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SynopsisRibbon-like lamellar single crystals of cellulose I1 are grown from dilute aqueous solution of cellulose triacetate (number-average degree of polymerization (DP)-15) by deacetylation and consequent precipitation. The best results are obtained with methylamine as the deacetylation reagent a t 90°C. Electron diffraction indicates the (170) planes to be the growth face. Twin growth occurs frequently with (170) as the twinning plane.

Lamellar single crystals of cellulose were obtained from dilute solutions of low‐DP cellulose triacetate, by deacetylation followed by precipitation. At temperatures between 150 and 160°C, pure cellulose IVII crystals were obtained whereas at temperatures between 90 and 150°C, hybrid crystals, having cellulose II and cellulose IVII domains cocrystallized in syntaxy were obtained. In both cases, the crystals were identified and characterized by electron diffraction. When the solutions leading to cellulose IVII were seeded with native cellulose microfibrils, a shish‐kebab structure resulted with the microfibrils decorated by cellulose II lamellae.

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