Amylose: structure

FIGURE 6.15 AFM images of (A) potato amylose, (B) potato amylopectin (arrows branch points on the chains), and (C) rice amylose (arrows individual amylose structures). Reprinted with permission from Dang et al. (2006). 

The reaction between starch and iodine (or iodine-iodide mixtures) to form an inclusion compound was first reported in 1814 by Colin and de Claubry 131) and has since become familiar to all chemists through its applications in analytical chemistry. Its deep blue colour (kmax 620 nm) has been known for years to result from a linear arrangement of polyiodide within a canal formed by a helical coil of amylose. The helical amylose structure will trap other molecules 132,1331 and other hosts will stabilise polyatomic iodide guests134> 135). 

While starch is the major structural polysaccharide, plant energy storage and regulation utilize a combination of similar polysaccharides that are referred to as starch. Starch can be divided into two general structures, largely linear amylose (structure 9.17) and largely branched amylopectin (structure 9.18). 

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... 

Classification of Amylose Structures and Their General Features... 

The unit cell dimensions of all crystalline amyloses that have been determined in some detail, are listed in Table I. Also included are some intermediate forms between the va and Vjj amyloses (Ji.) and some V-amylose complexes with n-butanol, which, although not yet completely determined, have been added to illustrate the range of variability in unit cell dimensions. In the case of the Va-BuOH complex, a doubling of one unit cell axis was detected after a careful study of electron diffraction diagrams of single crystals ClO). A consequence of the doubling is that the unit cell now contains four chains, instead of the two normally found in amylose structures. Cln a strict sense, the A- and B-amyloses should also be considered as four-chain unit cells, but their double-helical structure still results in only two helices per cell) (13,1 ). 

Almost all unit cells shown in Table I are either orthorhombic or pseudo-orthorhombic, with a majority of space groups P2] 2] 2] and P2. Only a few structures exhibit higher symmetry and none shows lower symmetry. All structures have an antiparallel packing of chains (however, see A- and B-amyloses). On the other hand, a large variety of helix characteristics are evident, in addition to the variability in the unit cell dimensions. Some of the features useful for classifying amylose structures are shown in Table II. The distance between the two nearest antiparallel-... 

All V-amylose structures shown in Table I have in common a left-handed, six-rgsidue helix, with h in a very narrow range from 1.32 to 1.36 A, and an 0-2...0-3(2) intramolecular hydrogen... 

Where the degrees of freedom are reduced, however, as in a cycloamylose with 26 residues, V-amylose structures can be observed in the absence of guests, at atomic resolution. The two helical regions in this molecule involve the left-handed helix with six residues per turn seen in the complex with glycerol it is possible to detect the 2-OH-3 -OH hydrogen bond between adjacent resi-dues, " " " and cross-helix bifurcated hydrogen bonds between 03 and 02 and... 

The amylose structure is somewhat different and is depicted below. 

It was found that the protected hexasaccharide forms homogeneous monolayer films with a density of molecules at the surface that is roughly one order of magnitude lower than that of alkanethiols. The concentration and time necessary to deprotect the layer at the surface, as well as the observed contact angle and angle-dependent XPS measurements, indicate contorted molecules at the surface (possibly the onset of the helical amylose structure) with consequent limited accessibility to the protective groups. 

---